Sébastien Voisembert
email: sebastien.voisembert@cea.fr

Nazih Mechbal
email: nazih.mechbal@paris.ensam.fr

Alain Riwan
email: alain.riwan@cea.fr

Ameziane Aoussat
email: ameziane.aoussat@ensam.eu

The aim of this paper is to present the design of a new long range robotic arm based on an inflatable structure. Inflatable robotics is on the horizon. It has potential for improved large payload-to-weight ratios, safe collision and inspection in areas inaccessible to human beings as in nuclear plants. The present robot is intended to operate inspection or maintenance missions in critical installation with taking care to not collide with its environment. It is made with innovative inflatable joints and an original actuation system. Prototypes of this inflatable manipulator were constructed using two different manufacturing procedures. Both were tested and numerically analyzed. The specific geometry of joints was studied and an analytical model for control purpose is proposed here

The experimentation results validated this new concept of robotic arm. Moreover, using LS-DYNA nonlinear dynamic finite element modeling we have analyzed the complete dynamical behavior of a joint. The simulations have given insight into understanding the joint bending process and have revealed guidance for optimizing the conception.

Introduction

Inflatable robots have become attractive because they can meet structural requirements for several expensive applications at a low cost. Moreover, their inherent abilities such as lightweight, large payload-to-weight ratios, and safe collision, have made them very appealing for specific manipulations as inspections operations in confined area or to operate in human environments. Nowadays, considerable effort is being devoted to the development and prototyping of inflatable manipulators.

Several new robots and actuators built out of fabric and powered via pneumatics or hydraulics have been developed. The first proposed inflatable manipulators were high degrees of freedom trump-like manipulators. As an example, the Octarm [START_REF] Trivedi | Optimal, Model-Based Design of Soft Robotic Manipulators[END_REF] is a bio-mimetic manipulator composed of inflatable actuators. It can handle objects with any kind of shape thanks to its highly compliant pneumatic muscles. Other manipulators were designed in a lightweight purpose with inflatable links and classical revolute joints [START_REF] Salomonski | Light robot arm based on inflatable structure[END_REF][START_REF] Koren | Inflatable Structure[END_REF][START_REF] Baldur | Inflatable manipulator[END_REF]. Sanan also created inflatable manipulators for safe interaction with humans [START_REF] Sanan | Robots with Inflatable Links[END_REF][START_REF] Sanan | Soft Robots for Safe Physical Human Interaction[END_REF]. Indeed, as they are not made of metals or carbon fibers like other classical robots, accidental contacts with operators are no longer causes of injuries. Furthermore, they are low-cost, lightweight and easy to store when deflated.

One of the major challenges in robotics is the improvement of inspections operations in hazardous area using unmanned remote handling systems. Articulated arms could be used in this case to carry some diagnostic tools for the inspection tasks.

These long reach multi-link carriers should be characterized by a large workspace and reduced mass. These serial tendon driven manipulators are mainly composed of a support base and a long range redundant serial arm made of links and joints.

They are widely used for inspection tasks in hostile environment such as nuclear industry, where a lot of cells to be inspected are only accessible through a very narrow hole. For example, in the case of maintenance operations in fusion reactors; particularly Plasma Facing Components viewing and leak testing inspections, the accessibility to the inspection area is only possible through some openings evenly distributed around the machine. Since 2005, the Interactive Robotics Laboratory of the CEA, the French Government Atomic Commission, has proposed and developed for the inspection of the vacuum vessel of Tokamak Fusion reactor Tore Supra and further ITER, a teleoperated arm named AIA (Articulated Inspection Arm). The AIA robot (Fig. 1) is an 8 m long multi-link carrier with payload up to 10 kg and a total weight of 150 kg. The polyarticulated arm can be introduced in the vessel through a small port of 250 mm diameter and should bear a 10 -6 Pa vacuum and 150°C temperature [START_REF] Keller | An ITER Relevant Robot for Remote Handling: on the Road to Operation on Tore Supra[END_REF]. On inspection mission the robot has to navigate around critical and fragile devices with taking care to not collide with them.

The arm should also have enough torque to carry its own weight plus the payload in cantilever mode and enough stiffness to minimize the deflection caused by the gravity. Moreover, for long reach we need to increase the number of joints and reduce the mass which therefore increases significantly the structural compliance. For a given material and with a constraint on the outer diameter, it can be shown that the deflection of a tube increases with its thickness.

In the Euler-Bernoulli beam theories, the deflection under its own weight of a cylindrical cantilever beam is given by:

I S E L g Y . 8 4    (1)
Where Y denotes the deflection at the end of the beam, g is the gravity,  is the density of the material, L is the length of the beam, E is the Young modulus of the material, S is the cross section area of the beam and I is the second moment of area.

If we introduce t as the thickness of the beam, we have

  2 2 1 1 1 . 2 R t R I S    (2)
Fig. 2: Influence of the thickness ratio on the deflections of a cantilever beam submitted to its own weight.

As shown in Fig. 2, a small thickness reduces the deflection. Unfortunately, for thin wall structures, the compressive strength is limited by buckling. This example brought out the question of how to increase compressive strength without adding weight? A solution is given by the concept of tensegrity where some elements bear compressive stress while other bear tensile stresses.

Hence, the robot body can be designed to be made of a light membrane or fabric, which bears only tensile stresses and another material, could be used to bear compressive stresses. The evident cheapest and lightest material that comes up in mind, in order to bear compressive strengths is air. In fact, the gas can be compressed inside the structure and pre-stress it. As a result, the buckling appears only when the compressive stress overtakes the pre-stress. In this study, a challenging design issue is addressed to improve significantly the operational range of these robots. We present here an innovative design of long range robots, named LRIA (Long Rang Inflatable Arm). The concept of tensegrity is used to make significant improvements on the weight and the cost of the arm. The proposed structure has the same overall shape as a classic multi-link arm, i.e. an alternation of links and joints. However, it is made of non-stretch fabric tightly coated. The benefits of such a robot are straightforward and obvious. For a light payload, i.e. 1 kg (for example, the weight of an inspection camera), the arm can reach very important length. Furthermore, it is easily movable when deflated and the setup can be rapid. Moreover, these arms can be fabricated and deployed with a drastic reduction of the cost according to conventional long range robots as for example the AIA robots-like. In this paper, we present new features of design and modeling of such a robot. We used a one-axis rotational joint with toroïdal bending at constant volume [START_REF] Koren | Inflatable Structure[END_REF]. It presents several benefits such as good stiffness in directions cross its rotation axis and low bending torque along its axis. This kind of joint was developed for spacesuits (Fig. 3) but with inner pressures lower than 30 kPa, whereas the minimum pressure needed by an inflated beam with the typical dimensions of a long range robot (10m long and 0.2m of diameter) that carries 1 kg is 130 kPa. Based on these statements, we present in the sequel two new designs of a higher pressure inflated structure convenient for very long range robots. Both designs have been tested on prototypes but not on the same material because of realization convenience. Thus in order to compare them, a dedicated finite element model has been developed. The finite element approach is also chosen as a simulation and design tool because of its ease of modeling complex geometries and ability to provide visualization. To take into account large displacements and membrane inflation, simulations were conducted using the explicit code LS-DYNA® (by LSTC). The simulations have given insight into understanding the bending process and revealed guidance to optimize the design. For control purpose of the robot, a study of the geometrics and kinematics of the inflatable joints must be driven. Therefore we present in this paper a first analytical model-based design of an n -DOF serial inflatable robot.

The remainder of this paper is organized as follows: Section 2 presents the finite element modeling that is used in the new designs evaluation in Section 3, and Section 4 deals with the numerical optimization of the tendon driven actuation system.

We conclude and present future works in Section 5.

Finite element study of the design

Explicit integration is often used for deployment simulations. It requires small time steps that are selected according to the complexity of the shape and the dynamics of the deployment. With the control volume (CV) approach available in LS-DYNA, the effect of inflation gas inertia is not directly included.

Presentation of the finite element model.

The finite element simulation aims at modeling accurately the shape of an inflatable arm according to our design. The objective is to predict numerically the behavior of a prototype after its inflation when submitted to actuation efforts. And more precisely, it aims at comparing the performances of two manufacturing procedures.

Simulation model

Finding the shape of an inflatable structure is now a solved problem but is still a large field of research for finite elements codes [START_REF] Tsiatas | Large Deflection Analysis of Elastic Space Membranes[END_REF][START_REF] Troufflard | Recherche de Forme des Gilets de Sauvetage Gonflables[END_REF][START_REF] Troufflard | Simulation Numérique de Recherche de Forme : Application aux Gilets de Sauvetage Gonflables[END_REF][START_REF] Breukels | Analysis of Complex Inflatable Structures Using a Multi-Body Dynamics Approach[END_REF][START_REF] Fokin | On Airbag Simulation in LS-DYNA with the Use of the Arbitrary Lagrangian-Eulerian Method[END_REF][START_REF] Diaby | Contribution à l'Etude du Flambement des Structures Gonflables[END_REF][START_REF] Davids | Finite-Element Analysis of Tubular Fabric Beams Including Pressure Effects and Local Fabric Wrinkling[END_REF][START_REF] Barnes | Form-Finding and Analysis of Prestressed Nets and Membranes[END_REF]. For examples, the form finding of inflated structures with very low shear modulus materials is still unsolved.

An inflated membrane raises several other mechanical problems such as large displacement calculation of membrane elements with no stiffness in flexion and self-contact [START_REF] Hughes | A Finite Element Method for a Class of Contact-Impact Problems[END_REF]. In order to take into account all the complexity of this problem we use membrane models and dedicated algorithms developed within the LS-DYNA ® finite element code by LSTC.

Here a prototype composed of two links and one central joint is discretized by finite elements. The links are 50cm long and the joint has theoretical amplitude of 90°. All the parts have the same diameter of 200mm (see Fig. 4). The fabric is represented by the material model *MAT_FABRIC [START_REF] Hallquist | LS-DYNA Theory Manual[END_REF][START_REF] Lstc | LS-DYNA Keyword User's Guide[END_REF], a four nodes element well adapted to airbags inflations.

Buckling is well represented even with a coarse mech. The mechanical properties chosen are consistent with the Dyneema® fabric used for the prototypes. An orthotropic model is applied with 11 GPa in warp and weft directions, which coincides with the longitudinal and transverse direction of the prototype. The shear modulus should be based on the properties of the polyurethane bladder but compared to the young modulus of the fabric, the value is too low to be computed. The minimum computable value is used instead.

The joint allows a rotation motion thanks to the pleats. The pleats modeling will be described in the next parts. The seams are assumed to be non-stretched so they are represented by some fabric elements stiffer than the rest of the fabric. The scenario of the simulation is twofold: first the virtual prototype is inflated to a pressure of 0.1 Mpa in 0.5 s to get the proper initial inflated shape. This phase is short because the inflation is not important in the simulation. Then the pleats are created and a constant bending force between the extremities of the joint is applied by a muscle element *MAT_SPRING_MUSCLE [START_REF] Hallquist | LS-DYNA Theory Manual[END_REF][START_REF] Lstc | LS-DYNA Keyword User's Guide[END_REF]. The muscle stands for the pulley blocks actuation system that will be presented in Section 4. It can be attached on several points of the joint to simulate different pulley blocks. This second phase is set to last 10 seconds in order to give enough time to the joint to bend. Gravity load is not applied.

Inflation model

The modeling treats the virtual prototype as a control volume, where the control surface is modeled by the fabric material.

The expansion is considered adiabatic and the ideal gas law is used. Algorithms within the LS-DYNA® finite element code are based on an airbag inflation model developed by Wang and Nefske [START_REF] Wang | A New CAL3D Airbag Inflation Model[END_REF][START_REF] Wang | Dynamic Deployement Simulations of Inflatable Space Structures[END_REF]. During all the simulation, the pressure, the volume and the temperature are recorded.

Contact model

Due to the complex shape of the joint, a consistent simulation must take into account the self-contact of the fabric. Indeed, experimentations show that when inflated, the excess of fabric in the joint (due to the folds) leads to the apparition of unexpected pleats.

Three contact treatments exist in LS-DYNA but all of them consider that one of the surfaces involved in the contact is a master surface and the other the slave surface. The master surface is not deformed by the contact efforts whereas the slave surface does. The most common is the penalty method; it consists in detecting penetrating nodes or edges and to apply a repulsive effort normal to the surface. This effort is calculated according to the elastic law of a virtual spring placed between the surface and the penetrating node. The stiffness of the spring depends on the mechanical properties of each element, its stiffness, its size, its surface and its bulk modulus. The value is given by (3):

where i k denotes the spring stiffness, si f is an adjustment factor set to 1, i K is the bulk modulus of the fabric, i A is the surface of the penetrated element and i V its volume. The second method is the kinematic constraint method. It does not allow penetration until there is a tension applied on the surface, the contact nodes are constrained by additional accelerations and velocities to stay on the contact surface. This way, the contact nodes follow the surface contact in its motion.

i i i si i V A K f k  (3)
The distributed parameter method is the last one: it is the same as the second one except that half the mass of the slave nodes is distributed on the master surface. Nevertheless, only the penalty method is implanted for self-contact problem then this is the method used for our simulations.

Three friction models are available in LS-DYNA, the sliding only contact, static and dynamic friction and viscous friction.

Static and dynamic frictions are based on the model of Coulomb, with a smooth interpolation between the static and dynamic friction coefficient given by: where denotes the coefficient of friction, is the dynamic coefficient of friction, is the static coefficient of friction, the relative velocity of the surfaces and is an arbitrary constant set by the user. The viscous damping is consistent with the Rayleigh model where the viscous damping matrix is given by multiplying the coefficient of viscous friction with the mass matrix.

In the simulations, only the Coulomb friction model is used. Two cases are performed in order to evaluate the effect of friction on the joint bending. In the first, the coefficients of friction are set to zero, and in the second, for simplicity, both static and dynamic coefficient of friction are set to the static coefficient of friction of the PVC (usual fabric coating) which value is 0.15.

Meshing

For all the following simulations, the meshing is composed of quadrangles. The link is similar to a simple inflatable beam which mechanical behavior is well known [START_REF] Comer | Deflections of an Inflated Circular-Cylindrical Cantilever Beam[END_REF][START_REF] Van | Bending and Buckling of Inflatable Beams: Some New Theoretical Results[END_REF][START_REF] Wielgosz | Deflection of Inflatable Fabric Panels at High Pressure[END_REF][START_REF] Wielgosz | Mechanics of Inflatable Fabric Beams[END_REF]. Then the meshing in the links is really coarse, elements are 50 mm long and 10 mm large. On the contrary, the meshing has been refined at the joint in order to represent accurately its behavior.

Nevertheless, if the elements are too small, calculation time becomes unacceptable. This is a well-known problem of explicit finite element calculation. According to the Courant-Friedrichs-Lewy law [START_REF] Courant | On the Partial Difference Equations of Mathematical Physics[END_REF], the time step cannot be smaller than the Courant time step which depends on the material stiffness and the size of the element. The meshing used is a compromise between a good accuracy and an acceptable time calculation. Furthermore, in order to simulate the orthotropic fabric, the elements are oriented along the longitudinal and the circumferential axis.

Validation

In order to validate the FE simulation, we have compared the deflections under vertical load of a real prototype with the deflections calculated by the modeling.

  v carb d s d e         (4)

Fig. 5: End point deflections calculated with the FE model and compared with experimental values

The prototype is composed of two links and one vertical axis joint inflated at 45000 Pa. One of the links is constrained and loads of 27 and 33 N are applied at the end of the second link for different pitch angles. In Fig. 5, the measured and calculated deflections are compared. The differences between the reality and the model can be explained by the poor quality of the prototype used in this experiments and the inaccurate shear modulus value. Nevertheless, this modeling is accurate enough to represent the behavior of inflated joints and make qualitative deduction concerning the design.

New designs of the inflatable arm 3.1 Design of a joint.

Here a 90° amplitude joint is taken as a design example. The inflatable joint must be able to change shape from a cylinder to a quarter of a torus with no volume variation. Hence, one side of the cylinder must extend until taking a toroïdal shape. The simplest way to reach this ability is to attach 2 portions of a torus by their middle line (Fig. 6). Unfortunately, this shape cannot be obtained from a 2 dimension fabric but can be approximated with a special confection. It consists in shortening the middle line of the joint by making pleats and sewing them together. Since the spatial conquest, this kind of joint has been used in space suits to allow astronauts moving in a suit stiffened by the internal pressure. However, this kind of joint has never been used in a robotic application. Sanan has proposed a simpler joint based on a section restriction and we have used a FE simulation to find out which joint best suits our long-range robot.

As our future robot is intended to work in the horizontal plane with gravitational loads in vertical plane, the joint we seek must be as compliant as possible along its rotational axis and as stiff as possible in cross directions. In the simulation, both designs are modeled and submitted to the same test: they are bent by a constant force of 90 N and a payload of 50 N is applied at the free extremity (Fig. 7). For our design, bellows are modelled by pleats on the flat fabric. Here the mechanical behaviour of both joints are compared i.e. the bending aptitude and the resistance to bending cross the rotation axis.

Compared to the Sanan's joint the bellow type joint exhibits twice the bending amplitude and is twice as stiff in flexion (Table1). This fact confirms that keeping the cross section constant along the arm increases dramatically its performance. Therefore the bellow type joint is selected for the following work.

Practical realization

The two principal functions the arm envelope should provide are air tightness and mechanical resistance to stress. Air tightness can be ensured by an inner tube while the mechanical requirement may be met by an independent fabric.

Nevertheless, assembling the fabric with a bladder may be difficult for a long robot arm. As a consequence, we propose to use a coated fabric to realise the robot. With this kind of fabric, both functions are fulfilled at the same time but the seams must be carefully over coated. In previous prototypes the pleats were sewed together but, with a coated fabric, this method is no longer valid because the seam must be planar in order to be over-coated. Basically, the purpose of the pleats is to shorten the middle line of the cylinder, but shortening this line may also be done without seams. Here, we propose and compare two manufacturing processes that allow building inflatable joints made of a tight coated fabric. Theoretically, both methods are effective but we have also tested their convenience and the actuation torque of the resulting joints with experiments and simulation.

Stop-pin based solution

The main difficulty is in the region where the length of the cylinder is shortened, all the tensions are concentrated and pull on the pleats, i.e. for 10 5 Pa in a 200 mm cylinder, the shortened lines (Fig. 8b) must bear a tension of 1570 N. Here, we propose an assembling method to realise pleats that can bear this tension. The idea is to constrain the pleats within a rigid shape.

Thus, we have used stop-pins as intern and extern shapes (Fig. 8a). The intern stop-pin keeps the fabric inside another stoppin which bears the tension. With this solution, the joints can be placed everywhere in the robot without seams. Furthermore, the joints are removable, which introduce modularity in the robot architecture.

Nevertheless, we have noted that intern stop-pins slowly slide out of the extern stop-pin until it is dangerously expelled by the fabric. This problem can be solved by replacing intern stop-pins by screws (Fig. 8c). The length g L of the stop-pin is an important parameter, obviously, if the stop-pin is to short the fabric cannot bear the stress concentration; and if it is too long the joint bending is limited.

Fig. 8 : Stop-pin principle (a). Joint middle line shortened with stop-pins (b). Joint with stop-pins and screws (c). Two axis prototype made with a PVC coated Polyester fabric (d).

This assumption is confirmed if we consider the stresses in the fabric between two stop-pins (Fig. 9). In the longitudinal direction, the fabric is submitted to the stress This simple modelling confirms that the torque needed to bend the joint is bigger with long stop-pins. Moreover, best performance should be achieved with zero-length stop-pins. This obviously, does not make physical sense because of the simplicity of this model. Therefore, a more refined model has been developed within the finite element code LS-DYNA in order to evaluate the influence of the stop-pin length.

The simulations consist in inflating the tube at 1.5×10 5 Pa and then creating the pleats. The stop-pins are composed of rigid bodies attached to the fabric. The pleats are created when the stop-pins get curved (Fig. 10). When the middle line is shortened, the fabric between the stop-pins is stressed (Fig. 11). The bending forces are applied by muscle elements located on the sides of the joint (see Fig. 11). During the simulation, the antagonist muscles are alternately tighten and loosen in order to bend the joint in positive and negative angles. This simulation is repeated with seven different stop-pins lengths; from 0 to 24.3 mm. For clarity, only 4 significant curves are displayed in Fig. 12. The actuation torque is calculated as the sum of the muscles tensions, counted positively for the muscle that produces positive angles and negatively for the other, multiplied by the cylinder radius.

The curves show a hysteresis phenomenon that has been also experimentally pointed out by Schmidt and Newman [START_REF] Schmidt | Modeling Space Suit Mobility: Applications to Design and Operations[END_REF] and Voisembert et al. [START_REF] Voisembert | A Novel Inflatable Robot with Constant Continuous Volume[END_REF], and is called the damping moment. Several studies [START_REF] Jin | A Six DOF Passive Robot System for Spacesuit Measurement[END_REF][START_REF] Schmidt | Modeling Space Suit Mobility: Applications to Design and Operations[END_REF][START_REF] Schmidt | An Investigation of Space Suit Mobility With Applications to EVA Operations[END_REF][START_REF] Newman | Modeling the Extravehicular Mobility Unit (EMU) Space Suit Physiological Impliacations for Extravehicular Activity (EVA)[END_REF] aim at measuring accurately this resisting force in space-suit joints in order to prepare extravehicular mission. It is commonly assumed that the hysteretic behavior of an inflated joint is due to the friction of the fabric fibers. But the numerical simulations show that even without friction modeled, this phenomenon exists. The physical cause of this hysteresis is still under investigation.

We can also observe that for most curves, there is a threshold which represents a minimum effort to apply in order to bend the joint. For short stop-pins, when the bending torque is superior to this threshold, the joint bends with almost no additional effort. The simulations show that a zero-length stop-pin does not provide better performances. The ideal length of the stoppins is a trade-off because the fabric wrap must be wide enough to bear the tension and thin enough to reduce the minimum bending torque. We propose another solution that overcomes this paradox in the next part.

Wire-based solution

The problem of stop-pin-based solution is that the tension is bear by the fabric itself, and then it must be wide to resist it. If the tension is bear by a wire, the problem is solved. As described in the following paragraph, the wire can not only be bound to the fabric at the extremities of the joint. In fact, when the joint is bent, the shortened middle lines are also submitted to a transverse effort. If we consider a dl length portion of wire, the static equilibrium (Fig. 13) is then expressed as follows:

0     y F x F T T y x B A (9)
x F and y F denote the components of F , the action of the fabric on the middle line. When  d (Fig. 13) is small, we can make the following approximation:

Middle line of the joint                           d T d T F d T d T F y x sin cos cos cos sin sin         (10)
    c t R T  (13)
The fabric applies also the lineal transverse tension t  on the middle line. As a consequence, the wire must be bound at its extremities in order to bear the longitudinal tension but also all the way down the middle line in order to support the transverse tension. Therefore, the wire is inserted through intermediate eyelets and bound to the final eyelets (Fig. 14a). We have tested this structure and it works on prototypes (Fig. 14b). We have also simulated its behavior (Fig. 14d) in order to compare with the previous design. The wire is modeled by springs and muscles elements. When the tube is inflated, the muscles retract and the middle line of the joint is shortened. As the wire supports the longitudinal tension, we can see in Fig. 15 that the fabric in the middle line is not stressed as with the stop-pins.

As a consequence, the wire reduces the risk of fabric tearing. We have also submitted the wire based joint to the same bending cycle as the stop-pin based joint. For cleaner figures, only the half bending cycle is displayed in Fig. 16. In order to compare the designs, the actuation threshold and the maximal amplitude are displayed in Fig. 17 for each stop-pin length and for the wire based joint. We can see that the wire-based joint has the widest amplitude and an actuation threshold similar to the best stop-pin based joint. Actually, using a wire instead of stop-pins to restrain the middle line is the better solution because it separates the problem of tightness from the problem of bearing concentrated tensions. The assembly precision is also improved because the cable is integrated on the planar fabric before the folds are created. Moreover, pulling the wire is a great deal easier than placing the stop-pins one by one on the fabric.

Comparison of the design and friction impact

When tested with and without friction, it appears that friction reduces the joint maximal pitch angle. Nevertheless, this effect represents only 10 % of the pitch angle for stop-pin based joints and 2 % for the wire based joints (see Fig. 19). The previous designs presented in [START_REF] Voisembert | Numerical Evaluation of a New Robotic Manipulator based on Inflatable Joints[END_REF] are more impacted (reduction by 70% of the maximum pitch angle) because the folds are in a flat configuration in which auto-contacts appears near the middle line with shear displacements (see Fig. 18). With the new designs the folds are not constrained and contacts are limited.

Actuation system

The actuation system must meet the specifications cited in Table 2. In order to add minimum weight, we propose a tendon based system with actuators at the base of the arm [START_REF] Voisembert | A Novel Inflatable Tendon Driven Manipulator with Constant Volume[END_REF]. The inflatable joints described in the previous section are basically planar continuously deformable sections. When actuated by tendons, it is possible to control the orientation of the end section, i.e. the joint angle [START_REF] Gravagne | On the Kinematics of Remotely-Actuated Continuum Robots[END_REF]. Nevertheless, the curvature can be changed by exterior forces with no possibility for the tendons to counter this effect (Fig. 20). We have called this a dislocation. For an inflatable joint, a dislocation is a non-toroïdal shape that keeps the volume constant. It can be characterized by the lateral displacement, , of the middle line (see Fig. 20c) and the maximal value is given by:

( ) √ (14)
In [START_REF] Voisembert | A Novel Inflatable Tendon Driven Manipulator with Constant Volume[END_REF] we have proposed and validated an actuation system based on pulley blocks that permits to control the bending angle and the shape of a planar continuously deformable inflatable joint. Provided that the pulley blocks are evenly placed and that every pulley block counts one more line than the next pulley (Fig. 21), the joint is forced to keep a toroïdal shape and the curvature is constant. For more details on the demonstration, see [START_REF] Voisembert | A Novel Inflatable Tendon Driven Manipulator with Constant Volume[END_REF].

In order to minimize the mass of the actuation system, we propose to guide the lines in the pulley blocks through simple rings. This method saves the mass of bearing pulleys but poses the problem of friction in the actuation system.

lines pulley block

One line pulley block

Fold One actuator

First we have assumed that there should be as many pulley blocks as folds to fully actuate a joint. For a high number of folds this assumption leads to use a high number of pulley blocks, which multiplies the sources of friction. In order to define to which extent the joint can be under actuated to reduce friction, we simulate a 90° joint with 9 folds and 1 to 9 pulley blocks (see Fig. 22b) and apply an exterior force. We repeat this simulation with two lateral forces. The relative dislocations is calculated by dividing the simulated dislocation by the theoretical maximal dislocation. The results are displayed in Fig. 23.

We can observe that having the same number of pulley blocks as of folds is not necessary. Four pulleys are enough to limit the dislocations of a 90° inflatable joint to 13% of the maximal dislocation.

Conclusion and future work

In this work, the design of a novel inflatable long-range arm is proposed. This low cost robot, gives new perspectives on several applications as maintenance in critical installation or safe human interaction. Joints of this original design are based on the principle of toroïdal bending at constant volume, which provides low actuation torques.

The comparison of a thin-wall beam with the performances of cutting-edge robots shows that an inflatable long range robot would answer the problem of deflection under gravity. Nevertheless, the deformations of the joints could be more significant than the structural deformations of the links. In future works we will analyze and model the mechanical behavior of the joint and define which could be the real performances of a long range inflatable robot.

This kind of joint is preferred to section-restriction joints because of its higher pitch angle and better torsion stiffness. Two manufacturing procedures for the inflatable joints with a tight coated fabric have been proposed and compared thanks to experimentation and finite element based simulations (LS-DYNA software). One is based on a stop-pin assembly, the other is based on a wire, and both purposes are to shorten the middle line of the joint without drilling the tight coat. The simulations show that both designs present a hysteretic behavior.

Hysteresis is a well known problem for these inflatable joints but is assumed to be caused by friction in the fabric. As a resisting force appears in the simulations, even without friction modeled, we can affirm that this phenomenon is not due to friction. It remains to investigate whether it is caused by "numerical damping", necessary to calculation stability, or by physical causes other than friction.

The comparative evaluation also shows that the wire-based joint has similar damping moment, and better pitch angle than the stop-pin based joint. Manufacturing experimentations have shown that the wire is easier to manipulate than the stop-pins and reduces the risks of fabric tearing. Nevertheless, even with a wire, the simulated damping moment is high and could be a problem for the tensile actuation system. Therefore, the damping moment will be investigated in order to better understand its origin and reduce its value.

Within a robotic application project, we have developed and numerically evaluated an actuation system based on pulley blocks. It reduces the dislocations, typical of continuously deformable joints, and keeps the curvature constant. As the pulley blocks are a great source of friction for the tendons, we have tested in simulation different numbers of pulley blocks and determined that a number of 4 is sufficient to reduce significantly the dislocations. In future works we will also test this assessment on prototypes.

The next development step will be to motorize the actuation system and define the control system.

Fig. 1 :

 1 Fig. 1 : AIA robot into a full vessel demonstrator (scale 1) [7].

Fig. 3 :

 3 Fig. 3: Spacesuit joints are composed of bellows [9].

Fig. 4 :

 4 Fig. 4: Finite element model of two links and one central joint.

Fig. 6 :Fig. 7 :

 67 Fig. 6: Ideal shape of an inflatable joint.

tFig. 9 :Fig. 10 :

 910 Fig. 9 : Fabric wrap between two stop-pins.

Fig. 11 :

 11 Fig. 11 : Von Mises stress (MPa) for a 13.5mm stop-pin joint.

Fig. 12 :

 12 Fig. 12 : Significant responses of stop-pin based joints to a full bending cycle in simulation.

Fig. 13 :

 13 Fig. 13 : Visualisation of the tensions applied on the middle line

d

  can be expressed with the local curvature radius   

Fig. 14 :

 14 Fig. 14 : Middle line of a joint with a loosen wire (a). Prototype with two wire-based joints (b). Shortening of the middle line of the joint (c). Simulated shortening of the middle line thanks to muscle elements (d).

Fig. 15 :Fig. 16 :

 1516 Fig. 15 : Von Mises stress (MPa) in the wire based joint.

  stop-pin and wire based joints to a half bending cycle in simulation

Fig. 17 :Fig. 18 :

 1718 Fig. 17: Comparative evaluation of stop-pin and wire based joint performances.

Fig. 19 :Fig. 20 :

 1920 Fig. 19: Friction impact on the maximum pitch angle.

  weight Lineal mass/lineal mass of the arm < 10%

Fig. 21 :

 21 Fig. 21: Pulley block system forcing the joint to keep a toroïdal shape

Fig. 22 :Fig. 23 :

 2223 Fig. 22: Simulated joints submitted to dislocations pulley blocks systems

Table 1 :

 1 Comparative evaluation of the joints

	Joint type	Max bending angle (°)	Deflection in cross direction (mm)
	Space-suit joint	81	11
	Section restriction	47	20

Table 2 : Actuation system specifications

 2


Damien Nouvel
email: damien.nouvel@univ-tours.fr

Jean-Yves Antoine
email: jean-yves.antoine@univ-tours.fr

Nathalie Friburger
email: nathalie.friburger@univ-tours.fr

Pattern Mining for Named Entity Recognition

Many evaluation campaigns have shown that knowledge-based and data-driven approaches remain equally competitive for Named Entity Recognition. Our research team has developed CasEN, a symbolic system based on finite state tranducers, which achieved promising results during the Ester2 French-speaking evaluation campaign. Despite these encouraging results, manually extending the coverage of such a hand-crafted system is a difficult task. In this paper, we present a novel approach based on pattern mining for NER and to supplement our system's knowledge base. The system, mXS, exhaustively searches for hierarchical sequential patterns, that aim at detecting Named Entity boundaries. We assess their efficiency by using such patterns in a standalone mode and in combination with our existing system.

Introduction

Named Entity Recognition (NER) is an information extraction task that aims at extracting and categorizing specific entities (proper names or dedicated linguistic units as time expressions, amounts, etc.) in texts. These texts can be produced in diverse conditions. In particular, they may correspond to either electronic written documents [START_REF] Marsh | Muc-7 evaluation of ie technology: Overview of results[END_REF] or more recently speech transcripts provided by a human expert or an automatic speech recognition (ASR) system [START_REF] Galliano | The ester 2 evaluation campaign for the rich transcription of french radio broadcasts[END_REF]. The recognized entities may later be used by higher-level tasks for different purposes such as Information Retrieval or Open-Domain Question-Answering [START_REF] Voorhees | International Speech Communication Association (INTERSPEECH'09)[END_REF]. While NER is often considered as quite a simple task, there is still room for improvement when it is confronted to difficult contexts. For instance, NER systems may have to cope with noisy data such as speech recognition errors or speech disfluences. In addition, NER is no more circumscribed to proper names, but may also involve common nouns (e.g., "the judge") or complex multi-word expressions with embedded NEs (e.g. "the Computer Science Department of the New York University"). These complementary needs for robust and detailed processing explain that knowledgebased and data-driven approaches remain equally competitive on NER tasks as shown by many evaluation campaigns. For instance, the French-speaking Ester2 and Etape evaluation campaigns on radio broadcasts [START_REF] Galliano | The ester 2 evaluation campaign for the rich transcription of french radio broadcasts[END_REF] has shown that knowledge-based approaches outperformed data-driven ones on manual transcriptions. However, despite their advantageous precision, symbolic systems need significant efforts when confronted to new NE types or when the system has to be adadpted to diverse mdalities (written vs oral transcripts). In this paper, we present an original approach, based on the adaptation of pattern mining techniques as a machine learning process (automatic training on corpora to reach a large coverage), while remaining in the framework of symbolic resources (extraction of intelligible rules of NE recognition). The performances of the resulting system (mXs) on the Etape French-speaking evaluation compaign shows that this novel approach bears comparison with standard machine learning techniques (CRFs). Besides, coupling this system with CasEN [START_REF] Friburger | Finite-state transducer cascades to extract named entities in texts[END_REF], our knowledge-based system, provides us with promising results.

In Sect. 2 we present and compare approaches for NER. Sections 3 and 4, we describe how lexico-syntactic patterns may be extracted from annotated corpora and used as a standalone system. Finally, Sect. 5 and 6 reports experimental results on French oral corpora.

Related Work

In the 90's and until now, several symbolic systems have been designed that, often, make intensive use of regular expressions formalism to describe NEs. Those systems often combine external and internal evidences [START_REF] David | Internal and external evidence in the identification and semantic categorization of proper names[END_REF], as patterns describing contextual clues and lists of proper names by NE categories. Those systems achieve high accuracy, but, as stated by [START_REF] Mikheev | Named entity recognition without gazetteers[END_REF], because they depend on the hand-crafted definition of lexical ressources and detection rules, their coverage remains an issue.

Machine learning introduced new approaches to address NER. The problem is then stated as categorizing words that belong to a NE, taking into account various clues (features) in a model that is automatically parametrized by leveraging statistics from a training corpus. Among these methods, some only focus on the current word under examination (maximum entropy, SVM) [START_REF] Borthwick | Exploiting diverse knowledge sources via maximum entropy in named entity recognition[END_REF], while others also evaluate stochastic dependencies (HMM, CRF) [START_REF] Mccallum | Early results for named entity recognition with conditional random fields, feature induction and web-enhanced lexicons[END_REF]. Most of the time, these approaches output the most probable sequence of labels for a given sentence. This is generally known as the "labeling problem", applied to NER.

Many approaches [START_REF] Nadeau | A survey of named entity recognition and classification[END_REF] rely on pre-processing steps that provide additional information about data, often Part-Of-speech (POS) tagging and proper names lists, to determine how to automatically tag a text, resulting in an annotated text. Some make use of data mining techniques [START_REF] Freitag | Boosted wrapper induction[END_REF][START_REF] Etzioni | Unsupervised namedentity extraction from the web: An experimental study[END_REF], but we are not aware of work that goes beyond the step of extracting patterns for NER: no model has emerged for using those patterns to recognize NEs.

In this paper, we propose a system that adapts text mining techniques to the NER problem. The benefits of text mining techniques are twofold. On the one hand, pattern mining techniques are data driven and may be combined with standard machine learning approaches. On the other hand, pattern mining allows to extract NER detection rules (e.g. transducers) which are intelligible for a human expert and can be used by a symbolic system. To the best of our knowledge, this way of combining symbolic and machine learning approaches is completely original in the framework of NER.

Besides, our pattern mining system, mXS, focuses on boundaries of NEs, as begin-

D

Sent.

Patterns from L I s 1

The american <pers> president Barack Obama </pers> has arrived in <loc> Moscou </loc>. s 2

There he has seen the former <pers> chancelor Michelle Bachelet </pers>. s 3

The <pers> president Dimitri Medvedev </pers> was not present on the beautiful <loc> square Vladimir Lenine </loc>.

Table 1: sentences from an annotated corpus ning or ending markers that we would like to be inserted at correct positions. To this end, we extract patterns [START_REF] Nouvel | Recognizing named entities using automatically extracted transduction rules[END_REF][START_REF] Nouvel | Reconnaissance des entités nommées par exploration de règles d'annotation[END_REF] that are correlated to those markers. Those patterns, casted as "annotation rules", are not constrained to necessarily recognize both boundaries of NEs. Basically, the system detects each boundary of NEs separately. This strategy is expected to present a more robust behaviour on noisy data such as ASR recognition errors or speech disfluences. They are evaluated as a standalone system or coupled with our existing knowledge-based system.

3 Mining Hierarchical and Sequential Patterns

Extracting Patterns

We use data mining techniques to process natural language. In this context, what is detected as a sentence will be considered as a sequence of items, precluding the extraction of patterns accross sentences. Two alphabets are defined: W , words from natural language, and M as markers, e.g. the tags delimiting NE categories (e.g. person, location, amount). The annotated corpus D is a multiset of sequences based on items from W ∪ M . Table 1 exemplifies this with W = {The, president, Obama, . . . } and M = {<pers>, </pers>, <loc>, </loc>, <time>, . . . , <org>, . . . }.

Like most systems, the mining process relies as a first step on linguistic analysis of input data. Those preprocessing steps extend the language W to W * by lemmatizing, applying a Part-Of-Speech (POS) tagger and recognizing expressions from lexical resources provided by the ProlexBase [START_REF] Bouchou | Prolexbase et lmf : vers un standard pour les ressources lexicales sur les noms propres[END_REF] database (890K entries). Those additional elements are inserted as a hierarchical representation of tokens: each may gradually be generalized to its lemma, POS or semantic type. For instance in Table 1, the pattern language contains items {arrive, see, VER, JJ, DET, NN, NPP . . . }. The POS tagger distinguishes common nouns (NN) from proper names (NPP). Note that we only keep semantic information for proper names, to avoid extracting patterns that would contain instances of proper names. Figure 1 illustrates how POS categories are organized as a hierarchy and what patterns may be mined through an example of a sequence. We exhaustively extract contiguous patterns over this language. For instance, in Fig. 1, patterns such as 'VER in <loc> NPP' or 'NPP </loc> with' are extracted. The hierarchy and the properties of sequential patterns allow to partially order them. This use of the hierarchy as a modular description of language enables fine-grained generalizations for items inside patterns: extracting accurate patterns relies both on the data that is mined and on specifying a relevant hierarchy for NER.

Filtering Patterns as Annotation Rules

We mine a large annotated corpus to find generalized patterns that co-occur with NE markers. As usual in data mining, we set thresholds during extraction based on two interestingness measures: support and confidence. The support of a pattern P is its number of occurrences in D, denoted by supp(P, D). The greater the support of P, the more general the pattern P. To estimate empirically how much P is accurate to detect markers, we calculate its confidence. A function suppNoMark(P) returns the support of P when markers are omitted both in the rule and in the data. Thus, the confidence of P is:

con f (P, D) = supp(P, D) suppNoMark(P, D) (1)
As we are only interested in patterns correlated with NE markers, we extract patterns containing at least one marker as rules. For instance, consider the rule R = 'the JJ <pers> NN NPP' in Table 1. Its support is 2 (sentences s 1 and s 2 ). But its support without considering markers is 3, since sentence s 3 matches the rule when markers (<pers> in rule and <loc> in "the beautiful <loc> square Vladimir") are omitted. Thus the confidence of R is only 2/3.

The collection of transduction rules exceeding minimal support and confidence thresholds is used as a knowledge-base. In practice, the number of discovered rules remains very large (especially when minimal support threshold is low). Thus, we decide to filter-out the redundant rules. We consider two rules to be redundant if they are related by a generalization relation and if they have same support: they cover same sequences in data. Over a set of redundant rules, we only select the most specific ones, that we will actually use for NER. Those are named "Annotation Rules".

NER using Annotation Rules

We now aim at determining when transduction rules should insert markers in texts. Using rules as features, we are able estimate probabilities for marker's presence as a trained model, as we will present in Sect. 4.1. The rules and the model are provided to a simple beam search algorithm described in Sect. 4.2 to actually annotate texts.

Estimating Likelihood of Annotated Sentences

As previously mentioned, instead of assigning a category to words (or tokens), our annotation rules insert markers at diverse positions in sentences. At a given position of a sentence, a decision should choose between adding any number of beginning or ending markers for NE categories (e.g. <pers>, </pers>, <loc>, etc.) or not to do so, what we denote by inserting a 'void marker" ( / 0). We train as many binary classifiers as distinct markers to estimate local probabilities for each individual markers. The probability of the presence of a single marker m ∈ M at a given position i is a random variable conditioned by the set of rules that have been triggered at current position:

P(m ∈ M i |R 1 , R 2 . . . R k ) that we note P(m ∈ M i ).
Combining those separate probabilities allows a direct computation for the probability of having multiple markers, as a multilabel problem:

P(M i = {m 1 , m 2 }) = ∏ m∈{m 1 ,m 2 } P(m ∈ M i ) ∏ m∈M -{m 1 ,m 2 } (1 -P(m ∈ M i )) (2)
Finally, we use those local probabilities of the sequences1 of markers (including the void marker / 0) to compute the likelihood of any annotation as n independent decisions over a sentence:

P(M 1 , M 2 . . . M n ) = ∏ i=1...n P(M i ) (3)
Indeed, what is considered as the most likely annotation within possible ones has to maximize that measure.

Decoding Step

The probability model may lead to an invalid sequence of markers according to the considered annotation scheme. The decoding algorithm must therefore consider only the valid proposals of the model. Depending on the dataset, we'll expect to be able to generate flat (no embeddings) or strutured xml-like (Etape) annotations. Those constraints are implemented into the sequential algorithm, as a simple grammar to be checked while decoding. The latter uses a beam search approach 2 , where adding a marker is considered as making a transition: for instance, inserting a <loc> marker at the beginning of a sentence moves from a "not inside any NE" state to a "in loc" state.

The algorithm starts from the beginning of a sequence and, for any position, generates new annotation hypothesis at this point by taking into account probabilities, possible marker sequences and annotation scheme constraints. Annotation hypothesis are then ordered and selected depending on their likelihood. Note that only the best hypothesis is kept for any given state, and that the number of states may be finite (flat annotation schemes) or infinite (when embeddings are allowed without depth restrictions). At the end of the sentence, the resulting annotation is the hypothesis that ends up with a "not inside any NE" state.

Ester2 and Etape French Evaluation Campaigns

Our system, mXs, has initially been developed for the Ester2 campaign. Sub-sections 5.2 and 5.3 present detailed experimental results conducted on this corpus. They give a better insight of the system behaviour and assess the influence of the support and confidence thresholds. Then, Sect. 5.4 presents the official performances during the Etape evaluation campaign, which was a follow up of Ester2.

Data: French Radio Transcripts

Our system participated to the Ester2 and Etape evaluation campaigns, which involved the French-speaking research community on the problematic of NER on radio transcripts. This task is much more challenging on this kind of noisy data, due to speech disfluences, speech recognition repairs and absence of sentence boundaries. This accordingly lowers performance of POS tagging and, at a higher level, requires a much more robust approach to find entities. The French Ester2 and Etape evaluation campaigns included NER on transcribed texts [START_REF] Galliano | The ester 2 evaluation campaign for the rich transcription of french radio broadcasts[END_REF]. The competing systems had to recognize persons, locations, organizations, products, amounts, time and positions. Entitites were manually annotated for evaluation purposes. As reported by [START_REF] Nouvel | An analysis of the performances of the casen named entities recognition system in the ester2 evaluation campaign[END_REF], the Ester2 reference corpus contains many annotation inconsistencies. This is why we have decided to re-annotate consistantly one half of the corpus. This gold corpus will be named Ester2-corr while the second part was held out (Ester2-held). Such inconsistencies were avoided in the Etape corpus. Its annotation scheme is an extended version of Ester2's: evaluation is fine-grained and substructures of NEs are annotated as "components" [START_REF] Galibert | Structured and extended named entity evaluation in automatic speech transcriptions[END_REF]. Quantitative characteristics of those corpora are presented in Table 2.

Annotation Rules Extraction for Ester2

Corpora Ester2-dev and Ester2-held are merged to extract patterns. We used Tree-Tagger [START_REF] Schmid | Probabilistic part-of-speech tagging using decision trees[END_REF] for robustly tokenizing, POS-tagging and lemmatizing words (on French written texts, this tool provides high accuracy, more than 90% but, as far as we know, no evaluation has been made over oral transcriptions). The mining task requires many optimizations [START_REF] Nouvel | Reconnaissance des entités nommées par exploration de règles d'annotation[END_REF] and we used a level-wise algorithm [START_REF] Mannila | Discovery of frequent episodes in event sequences[END_REF] which leverages the generalization over patterns to mine frequent ones. Table 3 reports the number of rules, the number of non-redundant rules and the gain (i.e., the ratio between the number of rules and that of non-redundant ones). This elimination of redundant rules leads to a very significant reduction without loss of information from train corpus, what is very important for using this collection as a knowledge-base.

mXS Performance for Ester2

To assess efficiency of patterns for NER, we use Ester2-dev and Ester2-held merged to extract patterns and learn model, Ester2-corr to evaluate accuracy of the predicted markers. We train as many binary classifiers as necessary, using extracted rules as features to feed the logistic regression algorithm of SciKit toolkit [START_REF] Pedregosa | Scikit-learn: Machine learning in python[END_REF] 3 . In order to retrieve a set of rules that covers as much as possible actual markers in texts, we hereby extract rules at low support (3) and confidence (0.3) thresholds. With this exhaustive set of rules, only 52 markers out of 5196 (1%) are undetectable by the model because no rules are triggered at the considered position. Table 4 presents global score Slot Error Rate (SER) [START_REF] Makhoul | Performance measures for information extraction[END_REF] for diverse support and confidence thresholds. Those are computed by counting typed errors: insertions, deletions, types, extents. Results show that, for any support threshold, the model obtains better results at low confidence: even very generic (and thus less confident) rules are to be Table 7: error differences on CasEN with extracted rules performances close to state-of-the-art systems when correctly set up. Our latest experiences that hybridates mXS and CasEN for Etape obtains 32.9 SER (compared with the performance of the best system, CasEN: 35.2). Generally, our experiments suggest that our system is efficient for combining other systems outputs, we plan to conduct more experimentation on this topic in future.

Conclusion

In this paper, we reported experimentations on the use of pattern mining techniques to automatically enrich a knowledge-based NER system. We implemented a prototype which extracts patterns correlated to NE markers. The system exhaustively looks for annotation rules from a training corpus and filters out those of interest. During the mining process, the text is represented as a sequence of items, which may be generalized using a hierarchy through POS categories, and where the beginning or ending markers of NEs may be separately mined. The quality of patterns and their potential to recognize entites has been assessed and allowed us to state which are the most efficient and what markers categories remain to be improved. These experiments also investigated the idea of separately evaluating the probability to begin or end an entity, a beam search being afterwards responsible for finding the most likely and valid annotation. The resulting system was used coupled with a symbolic system, showing significant improvement of the performance. This work provides us with some interesting directions to improve a symbolic NER system, including in its foundations.

Acknowledgement

We'd like to thank Springer for allowing authors to share this authored version of the article published with reference LTC 2011, LNAI 8387 (Chapter 19, 978-3-319-08957-7, 328129 1 En).

Figure 1 :

 1 Figure 1: POS hierarchy and example of tagged sentence

Table 2 :

 2 characteristics of Ester2 and Etape corpora

	Corpus Tokens Sentences	NEs
	Ester2-dev	73 386	2 491	5 326
	Ester2-held	48 143	1 683	3 074
	Ester2-corr	40 167	1 300	2 798
	Total 128 477	4 283	8 670
	Etape-train 355 975	14 989	46 259
	Etape-dev 115 530	5 724	14 112
	Etape-test 123 221	6 770	13 055
	Total 594 726	27 483	73 426

Table 3 :

 3 extraction over Ester2 corpus at support and confidence thresholds

Each set of markers is mapped to a predetermined number of corresponding markers sequences, e.g. P({m 1 , m

}) = P(< m 1 , m 2 >) = P(< m 2 , m 1 >) = P(< m 1 , m 2 , m 1 >)

It limits the search space by considering at any position N most probable solutions

With regularization parameter C=4.

included as potentially relevant features. Globaly, using those rules as a standalone system remains unsufficient compared to state-of-the-art, but opens up great possibilities for coupling.

mXS performance for Etape

The Etape evaluation campaign extends Ester2 by considering TV broadcasts (including debates) and adding both fine grained and recursivity: NER is more difficult and requires robust approaches. Besides, the systems are now expected to recognize "componants" inside NEs [START_REF] Galibert | Structured and extended named entity evaluation in automatic speech transcriptions[END_REF], e.g. first names and last names for persons, day, month and year for dates, etc. As reported in Table 5, mXS is ranked 4th after one knowledgebased and two data-driven systems. Results show that the former have better performance for entities, while the latter are more accurate for componants. Despite being a data-driven system, mXS exhibits a behaviour similar to knowledge-based ones. The detailed results by NE types of Table 6 indicates that our system obtains good results for product category, which is the most difficult one. Conversely, it seems less efficient for recognizing organizations, which are known to be very ambiguous [START_REF] Nouvel | Recognizing named entities using automatically extracted transduction rules[END_REF]. This points out that pattern mining is accurate for detecting new NE types, but lacks fine-tuning for more traditional ones. We aim at improving performances of the existing system, CasEN, with the extracted patterns. Our symbolic system is precise, but lacks coverage because it would have to describe all regular expressions that may constitute a NE. Our idea is that automatically extracted patterns may supplement the symbolic system. We test this coupling by making CasEN's output a feature provided to mXS's input. Table 7 reports the initial symbolic system's results, the differences of errors by NE categories and the resulting coupled system's performance. The symbolic system alone outperforms our standalone system using rules (28 vs 38 SER). By coupling systems, we observe a significant improvement of the symbolic system's output. The insertion of a small amount (2) of false-positive (Ins. total) is the counterpart for the correction of 26 type errors made by the symbolic system. This mainly concern amounts and the balancing between location and organizations (which are known to be very amibiguous).

We also isolated and manually examined rules that were responsible for the decrease of errors (coverage). Most of these rules are short and generalized rules, and quite frequently inserting only one marker (for instance 'from <pers> NPP NPP' or 'to <loc> NPP'). Interestingly, two time expressions have been found thanks to the separate detection of the beginning and the ending markers using local clues: 'for <time>' and 'years </time>' (recognizing "for a few years" for instance). How those shallow rules may be taken into account by the knowledge base of the symbolic system remains to be investigated.

Due to lack of space, we do not report other configurations and coupling strategies that has been successfully experimented, those are reported in [START_REF] Nouvel | Coupling knowledge-based and data-driven systems for named entity recognition[END_REF]. They achieve