Frictional Contact in Interactive Deformable Environments

L\u2019uso di simulazioni garantisce notevoli vantaggi in termini di economia, realismo e di flessibilit\ue0 in molte aree di ricerca e in ambito dello sviluppo tecnologico. Per questo motivo le simulazioni vengono usate spesso in ambiti quali la prototipazione di parti meccaniche, nella pianificazione e nell\u2019addestramento di procedure di assemblaggio e disassemblaggio inoltre, di recente, le simulazioni si sono dimostrate validi strumenti anche nell\u2019assistenza e nell\u2019addestramento ai chirurghi, in particolare nel caso della chirurgia laparoscopica. La chirurgia laparoscopica, infatti, \ue8 considerata lo standard per molte procedure chirurgiche. La principale differenza rispetto alla chirurgia tradizionale risiede nella notevole limitazione che ha il chirurgo nell\u2019interagire e nel percepire l\u2019ambiente in lavora, sia nella vista che nel tatto. Questo rappresenta una forte limitazione per il chirurgo a cui \ue8 richiesta una lunga fase di addestramento prima di poter ottenere la necessaria destrezza per intervenire in laparoscopia con profitto. Queste limitazioni, d\u2019altra parte, rendono la laparoscopia il candidato ideale per l\u2019introduzione della simulazione nell\u2019addestramento. Attualmente sono disponibili in commercio dei software per l\u2019addestramento alla laparoscopia, tuttavia essi sono in genere basati su modelli rigidi, o modelli che comunque mancano del necessario realismo fisico. L\u2019introduzione di modelli deformabili migliorerebbe notevolmente l\u2019accuratezza e il realismo delle simulazioni. Nel caso dell\u2019addestramento il maggior realismo permetterebbe all\u2019utente di acquisire non solo le conoscenze motorie basilari ma anche capacit\ue0 e conoscenze di pi\uf9 alto livello. I corpi rigidi, infatti, rappresentano una buona approssimazione della realt\ue0 solo in situazioni particolari ed entro intervalli di sollecitazioni molto ristretti. Quando si considerano materiali non ingegneristici, come accade nelle simulazioni chirurgiche, le deformazioni non possono essere trascurate senza compromettere irrimediabilmente il realismo dei risultati. L\u2019uso di modelli deformabili tuttavia introduce notevole complessit\ue0 computazionale per il calcolo della fisica che regola le deformazioni e limita fortemente l\u2019uso di dati precalcolati, spesso utilizzati per velocizzare la fase di identificazione delle collisioni tra i corpi. I ritardi dovuti all\u2019uso di modelli deformabili rappresentano un grosso limite soprattutto nelle applicazioni interattive che, per consentire all\u2019utente di interagire con l\u2019ambiente, richiedono il calcolo della simulazione entro intervalli di tempo molto ridotti. In questa tesi viene affrontato il tema della simulazione di ambienti interattivi composti da corpi deformabili che interagiscono con attrito. Vengono analizzati e sviluppati differenti tecniche e metodi per le diverse componenti della simulazione: dalla simulazione di modelli deformabili, agli algoritmi di identificazione e soluzione delle collisioni e alla modellazione e integrazione dell\u2019attrito nella simulazione. In particolare vengono valutati i principali metodi che rappresentano lo stato dell\u2019arte nella modellazione di materiali deformabili. L\u2019analisi considera i fondamenti fisici su cui i modelli si basano e quindi sul grado di realismo che possono garantire in termini di deformazioni modellabili e la semplicit\ue0 d\u2019uso degli stessi (ovvero la facilit\ue0 di comprensione del metodo, la calibrazione del modello e la possibilit\ue0 di adattare il modello a situazioni differenti) ma viene considerata anche la complessit\ue0 computazionale di ciascun metodo in quanto essa rappresenta un fattore estremamente importante nella scelta e nell\u2019uso dei modelli deformabili nelle simulazioni. Il confronto dei differenti modelli e le caratteristiche identificate hanno motivato lo sviluppo di un metodo innovativo per fornire un\u2019interfaccia comune ai vari metodi di simulazione dei tessuti deformabili. Tale interfaccia ha il vantaggio di fornire dei metodi omogenei per la manipolazione dei diversi modelli deformabili. Ci\uf2 garantisce la possibilit\ue0 di scambiare il modello usato per la simulazione delle deformazioni mantenendo inalterati le altre strutture dati e i metodi della simulazione. L\u2019introduzione di tale interfaccia unificata si dimostra particolarmente vantaggiosa in quanto permette l\u2019uso di un solo metodo per l\u2019identificazione delle collisioni per tutti i differenti modelli deformabili. Ci\uf2 semplifica molto l\u2019analisi e la definizione dei requisiti di tale modulo software. L\u2019identificazione delle collisioni tra modelli rigidi generalmente precalcola delle partizioni dello spazio in cui i corpi sono definiti oppure sfrutta la suddivisione del corpo analizzato in parti convesse per velocizzare la simulazione. Nel caso di modelli deformabili non \ue8 possibile applicare tali tecniche a causa dei continui cambiamenti nella configurazione dei corpi. Dopo che le collisioni tra i corpi sono state riconosciute e che i punti di contatto sono stati identificati e necessario risolvere le collisioni tenendo conto della fisica sottostante i contatti. Per garantire il realismo \ue8 necessario assicurare che i corpi non si compenetrino mai e che nella simulazione delle collisioni tutti i fenomeni fisici di interesse coinvolti nel contatto tra i corpi vengano considerati: questi includono le forze elastiche che si esercitano tra i corpi e le forze di attrito che si generano lungo le superfici di contatto. L\u2019innovativo metodo proposto per la soluzione delle collisioni garantisce il realismo della simulazione e l\u2019integrazione con l\u2019interfaccia proposta per la gestione unificata dei modelli. Una caratteristica importante dei tessuti biologici \ue8 il comportamento anisotropico, dovuto, in genere, alla loro struttura fibrosa. In questa tesi viene proposto un nuovo metodo per aggiungere l\u2019anisotropia al comportamento dei modelli massa molla. Il metodo ha il vantaggio di mantenere la velocit\ue0 computazionale e la semplicit\ue0 di implementazione dei modelli massa molla classici e riesce a differenziare efficacemente la risposta del modello alle sollecitazioni lungo le differenti direzioni. Le tecniche descritte sono state integrate in due applicazioni che forniscono la simulazione della fisica di ambienti con corpi deformabili. La prima delle due implementa tutti i metodi descritti per la simulazione dei modelli deformabili, identifica le collisioni con precisione e le risolve fornendo la possibilit\ue0 di scegliere il modello di attrito pi\uf9 adatto, dimostrando cos\uec la fattibilit\ue0 dell\u2019approccio proposto. La limitazione principale di tale simulatore risiede nell\u2019alto tempo di calcolo richiesto per la simulazione dei singoli passi di simulazione. Tale limitazione \ue8 stata superata in una seconda implementazione che sfrutta il parallelismo intrinseco delle simulazioni fisiche per ottimizzare gli algoritmi e che, quindi, riesce a sfruttare al meglio la potenza computazionale delle architetture hardware parallele. Al fine di ottenere le prestazioni richieste per la simulazione di ambienti interattivi con ritorno di forza, la simulazione \ue8 basata su un algoritmo di identificazione delle collisioni semplificato, ma implementa gli altri metodi descritti in questa tesi. L\u2019implementazione parallela sfrutta le capacit\ue0 di calcolo delle moderne schede video munite di processori altamente paralleli e ci\uf2 permette di aggiornare la scena ogni millisecondo. Questo elimina ogni discontinuit\ue0 nel ritorno di forza reso all\u2019utente e nell\u2019aggiornamento della grafica della scena, inoltre garantisce il realismo necessario alla simulazione fisica sottostante. Le applicazioni implementate provano la fattibilit\ue0 della simulazione della fisica di interazioni complesse tra corpi deformabili. Inoltre, l\u2019implementazione parallela della simulazione rappresenta un promettente punto di partenza per la realizzazione di simulazioni interattive che potr\ue0 essere utilizzato in ambiti di ricerca differenti, quali l\u2019addestramento di chirurghi o la prototipazione rapida.The use of simulations provides great advantages in term of economy, realism, and adaptability to user requirements in many research and technological fields. For this reason simulations are currently exploited, for example, in prototyping of machinery parts, in assembly-disassembly test or training and, recently, simulations have also allowed the development of many useful and promising tools for the assistance and learning of surgical procedures. This is particularly true for laparoscopic intervention. Laparoscopy, in fact, represents the gold standard for many surgical procedures. The principal difference from standard surgery is the reduction of the surgeon ability to perceive the surgical scenario, both from visual and tactile point of view. This represents a great limitation for surgeons who undergo long training before being able to perform laparoscopic intervention with proficiency. This, on the other hand, makes laparoscopy an excellent candidate for the use of simulations for training. Some commercial training softwares are already available on the market, but they are usually based on rigid body models that completely lack the physical realism. The introduction of deformable models may leads to a great increment in terms of realism and accuracy. And, in the case of laparoscopy trainer it may allow the user to learn not only basic motor skills, but also higher level capabilities and knowledge. Rigid bodies, in fact, represents a good approximation of reality only in some situations and in very restricted ranges of solicitations. In particular, when non engineering materials are involved, as happens in surgical simulations, deformations cannot be neglected without completely loosing the realism of the environment. The use of deformable models, however, is limited for the high computational costs involved in the computation of the physics undergoing the deformations and because of the reduction in pre computable data in particular for collision detection between bodies. This represents a very limiting factor in interactive environments where, to allow the user to interactively control the virtual bodies, the simulation should be performed in real time. In this thesis we address the simulation of interactive environment populated with deformable models that interact with frictional contacts. This includes the analysis and the development of different techniques which implement the various parts of the simulation: mainly the methods for the simulation of deformable models, the collision detection and collision solution techniques but also the modelling and the integration of suitable friction models in the simulation. In particular we evaluated the principal methods that represent the state of the art in soft tissue modeling. Our analysis is based on the physical background of each method and thus on its realism in terms of deformations that the method can mimic and on the ease of use (i.e. method understanding, calibration and ability to adapt to different scenarios) but we also compared the computational complexity of different models, as it represents an extremely important factor in the choice and in the use of models in simulations. The comparison of different features in analyzed methods motivated us to the development of an innovative method to wrap in a common representation framework different methodologies of soft tissue simulation. This framework has the advantage of providing a unified interface for all the deformable models and thus it provides the ability to switch between deformable model keeping unchanged all other data structures and methods of the simulation. The use of this unique interface allows us to use one single method to perform the collision detection phase for all the analyzed deformable models, this greatly helped during the identification of requirements and features of such software module. Collision detection phase, when applied to rigid bodies, usually takes advantage of pre computation to subdivide body shapes in convex elements or to construct partitions of the space in which the body is defined to speed up the computation. When handling deformable models this is not possible because of the continuous changes in bodies shape. The collision detection method used in this work takes into account this problem and regularly adapt the data structures to the body configuration. After collisions have been detected and contact points have been identified on colliding bodies, it is necessary to solve the collision in a physics based way. To this extent we have to ensure that objects never compenetrate during the simulation and that, when solving collisions, all the physical phenomena involved in the contact of real bodies are taken into account: this include the elastic response of bodies during the contact and the frictional force exerted between each pair of colliding bodies. The innovative method for solving collision that we describe in this thesis ensures the realism of the simulation and the seamless interaction with the common framework used to integrate deformable models. One important feature of biologic tissues is their anisotropic behavior that usually comes from the fibrous structure of these tissues. In this thesis we propose a new method to introduce anisotropy in mass spring model. The method has the advantages of preserving the speed and ease of implementation of the model and it effectively introduces differentiation of the model behavior along the chosen directions. The described techniques have been integrated in two applications that allows the physical simulation of environments populated with deformable models. The first application implements all the described methods to simulate deformable models, it performs precise collision detection and solution with the possibility to chose the most suitable friction model for the simulation. It demonstrates the effectiveness of the proposed framework. The main limitation of this simulator, i.e. its high computation time, is tackled and solved in a second application that exploits the intrinsic parallelism of physical simulations to optimize the implementation and to exploit parallel architecture computational power. To obtain the performances required for an interactive environment the simulation is based on a simplified collision detection algorithm, but it features all the other techniques described in this thesis. The parallel implementation exploits graphic cards processor, a highly parallel architecture that update the scene every milliseconds. This allows the rendering of smooth haptic feedback to the user and ensures the realism of the physics simulation. The implemented applications prove the feasibility of the simulation of complex interactions between deformable models with physics realism. In addition, the parallel implementation of the simulator represents a promising starting point for the development of interactive simulations that can be used in different fields of research, such as surgeon training or fast prototyping

Role of virtual simulation in surgical training

The comparison of the developments obtained by training for aviation with the ones obtained by training for surgery highlights the efforts that are still required to define shared and validated training curricula for surgeons. This work focuses on robotic assisted surgery and the related training systems to analyze the current approaches to surgery training based on virtual environments. Limits of current simulation technology are highlighted and the systems currently on the market are compared in terms of their mechanical design and characteristics of the virtual environments offered. In particular the analysis focuses on the level of realism, both graphical and physical, and on the set of training tasks proposed. Some multimedia material is proposed to support the analysis and to highlight the differences between the simulations and the approach to training. From this analysis it is clear that, although there are several training systems on the market, some of them with a lot of scientific literature proving their validity, there is no consensus about the tasks to include in a training curriculum or the level of realism required to virtual environments to be useful

GPU Based Physical Cut in Interactive Haptic Simulations

PURPOSE: Interactive, physics based, simulations of deformable bodies are a growing research area with possible applications to computer-aided surgery. Their aim is to create virtual environments where surgeons are free to practice. To ensure the needed realism, the simulations must be performed with deformable bodies. The goal of this paper is to describe the approach to the development of a physics-based surgical simulator with haptic feedback. METHOD:The main development issue is the representation of the organ behavior at the high rates required by haptic realism. Since even high-end computers have inadequate performance, our approach exploits the parallelism of modern Graphics Processing Units (GPU). Particular attention is paid to the simulation of cuts because of their great importance in the surgical practice and the difficulty in handling topological changes in real time. RESULTS: To prove the correctness of our approach, we simulated an interactive, physically based, virtual abdomen. The simulation allows the user to interact with deformable models. Deformable models are updated in real time, thus allowing the rendering of force feedback to the user. The method is optimized to handle high quality scenes: we report results of interactive simulation of two virtual tools interacting with a complex model. CONCLUSIONS: The integration of physics-based deformable models in simulations greatly increases the realism of the virtual environment, taking into account real tissue properties and allowing the user to feel the actual forces exerted by organs on virtual tools. Our method proves the feasibility of exploiting GPU to simulate deformable models in interactive virtual environments

Dynamics Simulation for the Training of Teleoperated Retrieval of Spent Nuclear Fuel

This paper addresses the problem of training of operators for telemanipulation tasks. In particular it describes the development of a physics based virtual environment that allows a user to train in the control of an innovative robotic tools designed for the retrieval of spent nuclear fuels. The robotic device is designed to adapt to very different environments, at the cost of an increased complexity in its control. The virtual environment provides realistic simulation of robot dynamics. The two most challenging tasks related to robot control have been identified and implemented in the simulation, leading to an effective tool for the training. The developed application is described in details and the outcome of one simulated intervention is proposed and analyzed in terms of user interaction and realism.JRC.E.8-Nuclear securit

Dynamics Simulation for the Training of Teleoperated Retrieval of Spent Nuclear Fuel

This paper addresses the problem of training of operators for telemanipulation tasks. In particular, it describes the development of a physics based virtual environment that allows a user to train in the control of an innovative robotic tool designed for the retrieval of spent nuclear fuels. The robotic device is designed to adapt to very different environments, at the cost of an increased complexity in its control. The virtual environment provides realistic simulation of robot dynamics. The two most challenging tasks related to robot control have been identified and implemented in the simulation, leading to an effective tool for the training. The developed application is described in details and the outcome of one simulated intervention is proposed and analyzed in terms of user interaction and realism

Computer assisted training for robotic surgery

description of a virtual reality simulator for robotic surger

Improving the development of surgical skills with virtual fixtures in simulation

This paper focuses on the use of virtual fixtures to improve the learning of basic skills for laparoscopic surgery. Five virtual fixtures are defined, integrated into a virtual surgical simulator and used to define an experimental setup based on a trajectory following task. 46 subjects among surgeons and residents underwent a training session based on the proposed setup. Their performance has been logged and used to identify the effect of virtual fixtures on the learning curve from the point of view of accuracy and completion time. Virtual fixtures prove to be effective in improving the learning and affect differently accuracy and completion time. This suggests the possibility to tailor virtual fixtures on the specific task requirements.Peer Reviewe

Improving the development of surgical skills with virtual fixtures in simulation

This paper focuses on the use of virtual fixtures to improve the learning of basic skills for laparoscopic surgery. Five virtual fixtures are defined, integrated into a virtual surgical simulator and used to define an experimental setup based on a trajectory following task. 46 subjects among surgeons and residents underwent a training session based on the proposed setup. Their performance has been logged and used to identify the effect of virtual fixtures on the learning curve from the point of view of accuracy and completion time. Virtual fixtures prove to be effective in improving the learning and affect differently accuracy and completion time. This suggests the possibility to tailor virtual fixtures on the specific task requirements.Peer Reviewe