IDMME 2002

Clermont-Ferrand, France , May 14-16 2002

ACTIVE CATHETER PROTOTYPING : FROM VIRTUAL TO REAL Christofer KÜHL ENS-Cachan Antenne de Bretagne, Campus de Ker Lann, 35170 Bruz, 02-99-05-52-71, [email protected]

Georges DUMONT IRISA et ENS-Cachan Antenne de Bretagne, [email protected]

Pascal MOGNOL IRCCyN et ENS-Cachan Antenne de Bretagne, [email protected]

Sébastien GOULEAU IRCCyN et IUT de NANTES, [email protected]

Benoît FURET IRCCyN et IUT de NANTES, [email protected] Abstract: In this paper, we focus on the global design of an active micro-catheter, which is fully in the scope of medical needs for near future, and on the electroformed realization of one of its shape memory alloy (SMA) actuators. Virtual prototyping is a powerful tool for designing manufactured objects and will be described here with respect to the goal application. In the medical domain indeed, one major demand is to dispose of active catheters to help in surgical exploration and interventions. For this reason, we develop, in collaboration with the Laboratoire de Robotique de Paris, such a micro catheter provided with micro actuators. This catheter has to be agile and thin enough to crawl its way into complex environment (human ducts). For this reason, we have chosen to use articulated links in its design. So we address this design by means of a virtual prototype especially developed, including the interaction with a duct model obtained by scanner and provided with an haptic interface. A description of an original simulation platform is also proposed. Then we present a part of the realization of the real prototype by studying, with plans of experiment, the machining of a Shape Memory Alloy spring, which will be used in the active part of the actuators. Résumé: Dans cet article, nous étudions le processus de conception d’un endoscope actif, répondant aux besoins futurs du milieu médical, et la réalisation de ses actionneurs en alliage à mémoire de forme (AMF) par électroérosion à fil. Le prototypage virtuel est un outil puissant de conception et sera décrit ici en regard de notre application. En effet, une demande majeure dans le milieu médical est de disposer d’endoscopes actifs afin d’apporter une aide dans les interventions chirurgicales. Dans ce but, nous développons en collaboration avec le Laboratoire de Robotique de Paris un tel endoscope équipé de micro-actionneurs. Cet endoscope doit être suffisamment agile et fin pour pouvoir évoluer dans un environnement complexe (conduits humains). Pour ces raisons nous avons choisi de développer un système polyarticulé. Nous améliorons par la réalisation d’un prototype virtuel incluant l’interaction avec un modèle de conduit issu de scanner, et dirigé par une interface haptique. La simulation s’appuie sur une plate-forme originale que nous décrivons. Nous présentons ensuite une partie de la réalisation du prototype réel à travers l’étude par plans d’expérience de la fabrication de ressorts en AMF qui seront utilisés pour actionner le système. Keywords: Virtual prototyping, Rapid prototyping, Endoscope, SMA

1 Introduction In the medical field, a strong demand is expressed by the surgeons to realize less invasive 1

IDMME 2002

Clermont-Ferrand, France , May 14-16 2002

inspection and operation devices. An answer to this problem is given by the micro-robotics systems which will enable the operating gesture to be assisted thanks to active endoscopes realization. The use of such systems can be spread into wide fields of applications as for example the inspection of ducts in the nuclear field. The CNRS demonstrator "micro inspect intra tube" demand gave us the opportunity to develop collaboration between various organisms: - The LRP (Laboratoire de Robotique de Paris) for the conception of an endoscope with distributed SMA (Shape Memory Alloy) actuators - The ENS (Ecole Normale Supérieure de Cachan – Antenne de Bretagne) and the IRISA (Institut de Recherche en Informatique et Systèmes Aléatoires) for the development of a simulator aimed at optimizing the developed prototypes - The IRCCyN (Institut de Recherche en Communication et Cybernétique de Nantes) for the realization of the SMA springs 2 Endoscopes There are several kinds of endoscopes: their application domain, their size and their morphology distinguish them. The flexible endoscopes (figure 1) are devices provided with optical fibres which convey the image up to an ocular. The ocular allows a direct vision or an adaptation of a camera. Their diameter varies from 5 to 15 mm. The head of this device is generally directional through an orientation Fig. 1 : endoscope with orientation system system. This one is constituted by four cables connected to the head of the endoscope and which are pulled by the surgeon by means of knurls. A new generation of active endoscopes was developed by Olympus [1]. The orientation of the head is controlled through flexion modules activated by SMA. However, these systems do not allow to adapt their curvature to the investigated environments. Certain number of prototypes is in progress of development to improve the navigation of endoscopes in the inspected ducts. A prototype developed in Japan [2] is established by trays interconnected by SMA threads (figure 2). The orientation between two trays connected by three SMA threads is controlled by piloting the length Fig. 2 : Multilink active catheter the above-mentioned thread. The prototype developed in the LRP [3] is a stiff polyarticulated structure (figure 3). The endoscope body possesses an outside diameter of 8 mm and its length is, due to its structure, indefinite. The mechanism structure 2

IDMME 2002

Clermont-Ferrand, France , May 14-16 2002

consists of a succession of modules articulated to each other by pin joints. These connections are alternatively oriented at 90° to allow a 3D motion of the structure. The endoscope head (still in state of conception) should contain a device allowing to obtain a multi-directional vision of the observed space. This device could use a prism actuated by a polymer gel. This system is usually protected by a metallic sheath in industrial endoscopes and/or by a flexible polymer sheath in medical endoscopes. On every link, two SMA springs are mounted in an Fig. 3 : polyarticulated endoscope antagonist configuration to change the relative orientation. The command is provided through an integrated circuit controlling the electrical power supplied to the SMA. The following study approaches on one hand the optimization of the conception and on the other hand the feasibility of the actuators. These two points were led collectively to validate, and the aptness of the choice of the polyarticuled structure, and the feasibility of springs by a means of simple manufacture (thread electroforming). 3 Study simulator 3.1 Interests of the simulator The simulator objective is to be able to navigate virtually in a patient body, from the reconstruction of the digestive tract obtained by a MRI. By using the techniques of virtual reality, the feeling of dumping will be maximal by using graphic and haptic interfaces. The interest of the simulator is triple: - Young surgeons training: the training on simulator is obviously more accessible, less expensive and less risky; - Preoperative training: the simulator will allow the surgeon to repeat virtually the task to be made on the patient to operate; - Virtual prototyping [4]: the simulator allows to test the tool to estimate its quality for a given task. So we try afterward to determine, by optimization techniques based on genetic algorithms, which is the best candidate to make this task, so as to propose to the surgeon an outstanding endoscope. The mechanical model of simulation used and the organization in an original platform of simulation is developed here. Then the interaction model between the endoscope and the inspected channel is presented. Then the control module of SMA actuators and the various reserved strategies of command are described. Some results of interaction between an artery model, directly obtained from a medical acquisition, and an endoscope model are outlined. 3.2 GASP platform The management of the simulator is made on the platform GASP [5]. It is integrated in the SIAMES project at IRISA. The main objective of GASP is to propose a modular simulation 3

IDMME 2002

Clermont-Ferrand, France , May 14-16 2002

able to be executed on various material configurations. The platform manages the synchronization and the exchanges of data between the co-operative processes even if the frequencies of calculation are different, insuring so a "dilated real time". It is developed by using the specificities of the object-oriented programming (figure 4). Simulation

CAD Camera Model Dynamic

Graphic Interface

Dynamo

User Command

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Contact Model Force Feedback

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Fig. 4 : GASP (General Animated Simulation Platform) We are going afterward to clarify the various modules of the simulator. 3.3 Dynamic module The dynamic equations are resolved by C++ libraries [6]. The constraints method is used. Each body keeps its six DOF. The connections are translated into term of geometrical constraints. The integrator calculates the efforts developed in the connections not to violate the geometrical constraints. 3.4 Interaction module The reaction calculation is insured by a compliance method, because the interaction is made between a stiff body (the segments of the endoscope on one hand) and a soft body (human tissues on the other hand). The first stage of the reaction force calculation consists in collision detection between the endoscope and the channel. The detection is simply made in a geometrical way in the simulator: the surface of segments is represented by interaction points and we determine if there is any collision for each point. In that case, the following effort is applied: r r rr r F = − k .dist.n − f .(v .n ).n (1) r where dist is the penetration depth and n the normal orientation. An experimental protocol will be defined to identify the parameters of the compliance model. The channel is defined by a set of facets. To Fig. 5 : distance cartography - slice detect a collision, the distances of an interaction point to each facet are calculated and the lowest one is the distance from the point to the tube. 4

IDMME 2002

Clermont-Ferrand, France , May 14-16 2002

This calculation consumes obviously a lot of time. So we have developed a pre-processing method to build distance cartography in the space. The figure 5 represents the distances calculated by interpolation from the data stored for a slice of the tube. 3.5 Endoscope command We developed a module defining a controller which commands the mechanical model. The objective is to pilot the orientation of the endoscope segments to allow an easier introduction in the virtual duct. It is based on the geometrical description of the connections between two neighboring segments as well as on the geometrical and behavioral characteristics of the SMA actuators [7]. On an example of an endoscope constituted by 15 segments for which we affect orders of 10 ° orientation, we show that we have a fast system (time of answer of 0.4s) and very precise because the orders are finally reached. 3.6 Command strategies The objective is to adapt the shape of the endoscope to the geometry of inspected ducts. We propose here two methods by analyzing their limits. 3.6.1 Trajectory follow-up The operator, thanks to the image supplied by the display system, can determine the trajectory to follow. From this trajectory (which we keep regularly passage points), we determine step by step the orders to look to each of the joints to approach in best the trajectory (figure 6). This strategy allows the endoscope to progress into the duct without having any contact (figure 7). i=2 Articulation i Nearest point

θ \ dist (artic, traj )mini 1

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3 4

5

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7

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Fig. 6 : Trajectory follow-up Fig. 7 : Simulation result However, this method is valid only if the inspected channel has a constant geometry. In the case of a surgical inspection, the way through the ducts is modified in position and shape during time (breath of the patient, heartbeat…). So the defined trajectory is not valid anymore. Then it is important to find another progress method. 3.6.2 Multi-agent approach The objective of this approach is to have an automatic conformation of the endoscope by contact detection [8]. The solution we use relies on the split of the steering mechanism into independent subsystems and by considering them as agents. This is a very simple and modular solution independent of the length of the structure. Moreover, it is a strictly distributed approach minimizing the quantity of information exchanged between the agents. The figure 8 shows the answer of a structure constituted of two segments agents. When an effort is sensed, an order is 5

IDMME 2002

Clermont-Ferrand, France , May 14-16 2002

affected to the previous joint so as to decrease the interaction effort, the opposite order is affected to the next joint approval in a way that the continuation of the endoscope becomes an unchanged orientation. In three dimensions, the agents consist of four joints because of the alternated orientation of the pin joints. So, a sensed effort activates an order distributed on both previous joints according to its orientation, and the opposite orders set on the following joints. The figure 9 shows an endoscope piloted with this strategy. Agent i

Agent 1

Agent i-

Agent

Agent n

Fig. 8 : Multi-agent approach – 2D principle

Fig. 9 : Simulation result This method is particularly well adapted in inspections of human networks and allows, even if there are contact zones, to strongly limit the importance of the interaction efforts. 3.7 Insertion in a medical database From a patient scanning, a voxelised file describing the network shape is obtained. A supplementary computer processing is made to reconstruct an image of the inspected duct. The figure 10 represents an endoscope advancing in an environment recreated from a medical database.

Fig. 10 : Insertion in a medical data base 3.8 Conclusion We developed a simulator leaning on a complete mechanical description of an endoscope model piloted by SMA actuators and the inspected network. The application of the techniques of virtual reality, thanks to adapted graphic and haptic interfaces, will allow us to increase the feeling of dumping of the operator during the simulation. An important work is put in the application of the techniques of virtual prototyping. These

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IDMME 2002

Clermont-Ferrand, France , May 14-16 2002

aim at improving the quality of the developed prototypes. We shall use for this genetic algorithms (figure 11). A first study in 2D channels allowed to show the feasibility of this method: on this example (figure 12), the depth of penetration is optimized by considering the segment lengths as variables. This method should be widened to a 3D-duct model by using the results of dynamic simulation to estimate the objective function. The calculation time will be obviously a strongly punishing factor which will oblige to limit at the most the investigation domain and to make evaluations with various complexity levels stages (purely geometrical analysis, dynamic analysis…). ALGORITHM (1) RANDOM INITIALIZATION IF ( ! CONDITION) DO : (2) EVALUATION (1) Population

(6) Replacing

(2) Evaluation

(5) Mutation

(3) Selection

(4) Crossover

(3) SELECTION (4) CROSSOVER (5) MUTATION (6) REPLACING

Fig. 11 : genetic algorithms

Fig. 12 : optimized penetration depth

4 Manufacturing of SMA springs. A research about the prototypes manufacturing parts is developed. Our field of research takes into account the following means of realizations: - milling or turning machines (HSC or not); - EDM; - Rapid prototyping in order to obtain molded or injected parts. Our aim is to study these different processes to choose, according to the part (material, functions, forms), the most adapted process. To obtain the SMA spring, ours choices went on the characterization of the following processes: micromilling, EDM and molding. In this paper, we propose a method used to define correctly the cutting parameters to manufacture SMA springs. 4.1 Metallurgical problems EDM is a process which damages strongly the manufactured surface. The thermally affected layer, called white layer, is a zone in which the material have intense metallurgical alterations. These alterations can influence the mechanical characteristics. The scale on which we work increase this phenomenon significantly. So S. Henein from EPFL [9] was confronted with this problem during the realization of a spring with thin blade of 50 µm thickness manufactured by EDM (figure 13). Fig. 13 : blade spring 7

IDMME 2002

Clermont-Ferrand, France , May 14-16 2002

This spring is 8 mm thick; its roughness is 0,73mm. After polishing and attack in the acid, the thickness of the white layer was estimated between 1 mm to 2,5 mm. The researchers observed that there was 36 % difference of steepness between the theoretical one of the spring calculated and the real one of the manufactured part. Another major problem comes from the nature of the manufactured material and the finished part obtaining mode. SMA have thermodynamics properties: a sample of SMA deformed plastically at "low" temperature covers its initial shape by heating. The deformation at "low" temperature can affect 8 % in the case of Ti-Ni [10]. This memory effect is due to a reversible structural transformation in the solid state occurring between the "high" and the "low" temperature. A difference from 40 to 60 °C is sufficient between these temperatures and it is possible to choose the transformation temperature in a wide range by adapting the concentrations of the different components as well as by treating the elaborated material. If the material have, during the manufacturing, a too important thermal treatment, the SMA post-treatment could be impossible to realize. 4.2 Taguchi method The first study was to determine the surface reaction of the Nickel - Titanium being manufactured by EDM. There is no appropriate technology (cutting parameters) for the SMA manufactured parts. On the other hand, the Carbide and the Nitinol have close specific conductivity so we decided to use the technologies employed by FANUC for the carbide manufacturing parts. The manufactured spring is Fig. 14 : manufactured spring shown on the figure 14 (scale1). The spring is 0.2 mm thick. We can observe the thermally affected layer by the passage of the thread (figure 15). Thanks to a microscope, we have been able to estimate the thermally affected layer’s thickness which was included between 15 and 25 µm. This variation of thickness is probably due to the discontinuous evolution of the cutting parameters evolving during the manufacturing. The electric current delivered by the generator and the movements Fig. 15 : the thermally affected layer of the machine are enslaved by the tension of the thread. It allows to control in real time the cutting scale 25x et 50x conditions, but it modifies the thickness of the thermally affected layer. This thermally affected layer (white layer) results partially from a local melting of the material which was not evacuated by the dielectric. The figure 16 shows us an outline of the obtained state of surface. One can notice on this photo that the thickness of the zone is far from being constant. The peak which one perceives is due to a bad grip of the part (the part yields at the Fig. 16 : Spring, scale x5 8

IDMME 2002

Clermont-Ferrand, France , May 14-16 2002

end of manufacturing and eventually fallen) but now we know how to avoid this defect by better gripping the spring before it falls. To keep intact the proprieties of the SMA, we have to strongly reduce this thermally affected layer. We studied the relations between the cutting parameters and the thermally affected layer. We supposed that the roughness is directly proportional to the thickness of this layer. The objectives of this study are the following: - To study, through the parameters of roughness and speed machine, the relations between the machine’s parameters and the performances of the manufacturing; - To find the optimum cutting parameters to manufacture titanium part; - To determine a mathematical model allowing to represent the performances of the manufacturing according to the machine parameters. The adjustable cutting parameters on the EDM are: - The electric parameters: the vacuous tension (VS), the cutting tension (VM), the servo voltage (SV), the active time (ON), the time of break (OFF) and the resistivity (Res); - The mechanical parameters: the tension of the thread (T), the speed of the thread (VF), the pressure of the water (FR) and the machine’s speed (SPD). The execution of this plan, not fractional, including all these factors and having only two levels, would be 1024 experiments. With the Taguchi method, we set up a plan called 2 16 with only 16 experiments. The graph (figure 17) shows our experiments results. The factors are detailed one by one; one sees appearing the average of the factor and the effects associated to every level of the factors. 6

Res 5

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Fig. 17 : Graph of the effects for the speed This Taguchi method allowed us to set up quickly a set of cutting parameters allowing us to realize SMA springs. 4.3 Manufacturing of SMA springs The test to manufacture by EDM allowed us to put in evidence the thermally affected layer problems. The experiments are based on Taguchi method. With the results, we are now able to define the influential parameters and to envisage the mastered manufacture [11].

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IDMME 2002

Clermont-Ferrand, France , May 14-16 2002

5 Conclusion Virtual prototyping of active endoscopes is based on the control by an optimization algorithm of a dynamic simulation module in interaction with vessels. Thus the simulator is well defined, the interfaces and the optimization procedures should be improved. The manufacturing process study specific for the SMA by the plans of experiment enabled us to choose a satisfactory manufacturing method for the first prototype attempts. A study should be led to determine the impact of the thermally affected zone on the endoscope functioning. This double method of prototyping from virtual to real is a first step towards a method of concurrent engineering. This co-operation between conception and realization is our main goal in order to decrease development time. References [1] OLYMPUS. "Apparatus for bending an insertion section of an endoscope using a SMA", United States Patent, Patent Number 4,930,494, 1990. [2] K. PARK, M. ESASHI. "An active catheter with integrated circuit for communication and control", Microelectromechanical Systems (MEMS), 12th international IEEE, pp. 400-405 [3] J. SZEWCZYK, V. DE SARS, P. BIDAUD, G. DUMONT. "An active tubular polyarticulated micro-system for flexible endoscopes", Proceedings of ISER2000, 2000. [4] P. FUCHS, F. NASHASHIBI. “De la CAO à la réalité virtuelle”, Revue de CAO et d’informatique graphique, Vol. 13, 1998, pp. 131-167. [5] S. DONIKIAN, A. CHAUFFAUT, T. DUVAL, R. KULPA. “GASP: from modular programming to distributed executions”, Computer animation’98 IEEE. [6] B. BARENBURG. “Designing a class library for interactive simulation of rigid bodies”, Ph.D. Thesis, Eindhoven University, 2000. [7] N. TROISFONTAINE, P. BIDAUD. “Position and Force control of SMA micro-actuators”, International advanced robotics program, Vol. 10, pp. 110-126. [8] D. DUHAUT. “Using a multi-agent approach to solve the inverse kinematics”, Intelligent Robot and System Conference, IROS, 1993, pp. 2002-2007. [9] S. HENEIN, S. BOTTINELLI, R. CLAVEL. “Parallel spring stages with flexures of micrometric cross-sections”, Ecole Polytechnique Fédérale de Lausanne, SPIE, Vol. 3202, 1998, pp. 209-220. [10] E. PATOOR, M. BERVEILLER. “Les alliages à mémoire de forme”, Ed. Hermes. [11] S. GOULEAU. “Détermination des paramètres d’usinage par micro-électroérosion de pièces en alliage à mémoire de forme pour des applications dans le domaine de la micro-endoscopie”, Université de Nantes, DEA de Génie Mécanique, 2000.

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