Bert Willaert

In vivo soft tissue damage assessment for applications in surgery

In robotic and conventional minimally invasive surgery the risk of complications caused by collateral tissue damage remains high. This paper studies the concept of imposing damage thresholds on surgical instruments to avoid tissue overload. More specifically, the correlation between mechanical loading and damage in case of vascular clamping is investigated. With a computer controlled device, a high and a low clamping load were applied in vivo on the abdominal aorta of 43 rats. Samples of both loading levels were compared with zero load control samples and with samples clamped by a mosquito clamp w.r.t. functionality and histological integrity. Analysis of the samples shows that high clamping forces result in endothelial and smooth muscle cell destruction. Clamping with a mosquito clamp will cause even more damage to the elastic lamellae. Samples loaded at the lower load showed significantly less smooth muscle cell damage and a lower degree of endothelial damage. This paper is the first to statistically quantify the correlation between the degree of mechanical loading and the degree of tissue damage, thus setting the first steps towards tissue overload prevention during surgery. Future experiments will also include the effects of loading duration, recovery and patient-specificity.

Correlation between compression, tensile and tearing tests on healthy and calcified aortic tissues

An anastomosis performed in calcified tissues tears up faster than in healthy tissues. This study develops and validates an in vitro non-destructive method to distinguish healthy from calcified aortic tissues. An uniaxial unconfined compression test is able to distinguish healthy from calcified aortas (p<0.01). The compressive E-modulus at a strain level of 10% is 227+/-34kPa for artificially calcified and 147+/-15kPa for healthy porcine aortic tissues. Calcified aortic tissues have a lower tensile strength than healthy porcine aortic tissues (p<0.05). The ultimate tensile strength is 1.34+/-0.18MPa and 1.55+/-0.31MPa for artificially calcified and healthy porcine aortic tissues respectively. Calcified aortic tissues have a lower resistance to tearing than healthy aortic tissues (p<0.05). The resistance to tearing is 1.78+/-0.33N/mm and 2.16+/-0.64N/mm for artificially calcified and healthy porcine aortic tissues respectively.

A mechatronic analysis of the classical position-force controller based on bounded environment passivity

Bounded Environment Passivity, presented in this paper, allows one to design teleoperation systems that behave passively provided that the environment with which interaction takes place belongs to an a priori defined range of environments. The use of such a priori knowledge on the environment reduces conservativeness with respect to classical design approaches. An additional advantage lies in its capability to get a clearer insight on which type of environments are problematic for the specific controller under investigation. On the basis of a case study, i.e. the well-known Position-Force controller, this paper describes and compares different passivity-based methods. First, the traditional methods of two-port passivity and absolute stability are applied. The restrictions of these methods to come up with useful design rules are explicitly demonstrated. Second, the Bounded Environment Passivity conditions of the Position-Force controller are derived. These conditions describe the relation between the specific controller implementation, the teleoperator dynamics and the environment characteristics. In addition, the effects of structural resonance frequencies and low-pass filters, often present in realistic teleoperator setups, are described. This analysis reveals fundamental mechatronic rules of thumb for the design of a teleoperator system with a Position-Force control architecture. The theoretical results are verified experimentally on a one-degree-of-freedom teleoperation system.

Bounded environment passivity of the classical Position-Force teleoperation controller

This paper derives analytic guidelines to tune the popular Position-Force bilateral controller and improve its performance by incorporating available knowledge on the bounds of the environment impedance. The proposed guidelines can prove especially useful in the domain of telesurgery where a need exists for well-understood bilateral teleoperation controllers, that show good performance and where many tasks can be characterized by restricted and relatively easily definable impedance regions. This paper firstly analyses the two-port passivity and absolute stability properties of two alternatives of the Position-Force controller. The limitations on achievable performance when guaranteeing absolute stability with arbitrary environments are detailed. Next, a novel method, called Bounded Environment Passivity method is introduced. This method enables the design of teleoperation controllers that show passive behaviour for interactions with an environment that varies over a given range of impedances. A set of guidelines that allow a smarter trade-off between performance and stability follows. The theoretical results are verified experimentally on a 1-d.o.f. teleoperation setup.

Stability of model-mediated teleoperation: discussion and experiments

The design of a bilateral teleoperation system remains challenging in cases with high-impedance slave robots or substantial communication delays. Especially for these scenarios, model-mediated teleoperation offers a promising new approach. In this paper, we present a first stability discussion. We examine the continuous behavior using general control principles and discuss how the model structure and its predictive power affects system lag and stability. We also recognize the unavoidability of discrete model jumps and discuss measures to isolate events and prevent limit cycles. The discussions are illustrated in a single degree of freedom case and supported by single degree of freedom experiments.

Bilateral Teleoperation: Quantifying the Requirements for and Restrictions of Ideal Transparency

The well-known tradeoff between transparency and stability challenges the realization of bilateral teleoperation systems. Ideal conditions for the four-channel framework have been presented, but the practical feasibility is hardly quantified. This brief bridges the gap between existing detailed analysis of basic two-channel controllers and the ideal conditions for the four-channel framework. Starting from the fundamental stability and transparency limits of the Position-Force controller, two extensions of this controller have been analyzed in detail, using existing metrics for transparency and the absolute stability and bounded environment passivity criterion. This analysis quantitatively describes the effect of hardware properties, modeling errors, and low-pass filters and illustrates as such the restrictions on the realization of ideal transparency. The experimental results, presented in this brief, demonstrate the restricted stiffness transparency of the Position-Force controller, as well as how the extensions provide close to perfect stiffness transparency.

Towards Multi-DOF model mediated teleoperation: Using vision to augment feedback

In this paper, we address some of the challenges that arise as model-mediated teleoperation is applied to systems with multiple degrees of freedom and multiple sensors. Specifically we use a system with position, force, and vision sensors to explore an environment geometry in two degrees of freedom. The inclusion of vision is proposed to alleviate the difficulties of estimating an increasing number of environment properties. Vision can furthermore increase the predictive nature of model-mediated teleoperation, by effectively predicting touch feedback before the slave is even in contact with the environment. We focus on the case of estimating the location and orientation of a local surface patch at the contact point between the slave and the environment. We describe the various information sources with their respective limitations and create a combined model estimator as part of a multi-d.o.f. model-mediated controller. An experiment demonstrates the feasibility and benefits of utilizing vision sensors in teleoperation.

Transparent and Shaped Stiffness Reflection for Telesurgery

The motivation behind the work presented in this chapter is minimal access telesurgery with reliable haptic feedback. Minimal access surgery (MAS) is performed via small incisions in the body, through which long rigid instruments are inserted along with a camera. Telesurgery refers to surgical operations performed by robotic instruments, the slave, commanded by a surgeon through one or more complex robotic joysticks, the master. Nowadays, the only commercially available master-slave system for minimal access surgery is the Da Vinci Surgical System, which is frequently used for MAS (Hockstein et al., 2007). The benefits of telesurgical laparoscopy over conventional laparoscopy include increased number of degrees of freedom, elimination of tremor, 3D visualization, possible motion scaling and an ergonomic position for the surgeon (Corcione et al., 2005; Nguan et al., 2008). Moreover, the master-slave concept enables the surgeon to be outside the direct environment of the patient. In 2001 e.g., the first transatlantic surgical operation, a laparoscopic cholecystectomy, was performed on a patient in Strasbourg, France by a surgeon situated in New York, United States (Marescaux et al., 2001). Unfortunately, current telesurgical systems do not provide haptic feedback, which means that the surgeon loses his/her sense of touch. This decreases the efficiency of the surgeon and can result in collateral tissue damage (Bethea et al., 2004; De et al., 2007; Famaey et al., 2009). Several studies have shown that haptic feedback would be able to increase the precision of telesurgery and lower the interaction forces with the tissue (Deml et al., 2005; Tholey et al., 2005; Wagner et al., 2002). However, achieving a system with haptic feedback that accurately represents the feeling of soft tissue, while maintaining stability under all circumstances, is nontrivial and remains a big challenge. A first issue, which is not discussed in this chapter, concerns the design of a robust accurate force-measurement system that integrates well in the surgical environment (Peirs et al., 2004; Seibold et al., 2005; Willaert et al., 2009a; Zemiti et al., 2006). A second issue concerns the control itself of such a master-slave system. During MAS, the environment of interest consists mainly of soft tissue although also interactions with hard contacts can occur (i.e. contact with bone or another instrument). Based on the raw data acquired by Walraevens et al. (2008) (cardiovascular tissue) and by Rosen et al. (2008) (abdominal organs) one can estimate that the maximum stiffness is in the order of 1500 N/m when interacting with soft tissue. When contacting bone, the stiffness can be 13

A constraint-based programming approach to physical human-robot interaction

This work aims to extend the constraint-based formalism iTaSC for scenarios where physical human-robot interaction plays a central role, which is the case for e.g. surgical robotics, rehabilitation robotics and household robotics. To really exploit the potential of robots in these scenarios, it should be possible to enforce force and geometrical constraints in an easy and flexible way. iTaSC allows to express such constraints in different frames expressed in arbitrary spaces and to obtain control setpoints in a systematic way. In previous implementations of iTaSC, industrial velocity-controlled robots were considered. This work presents an extension of the iTaSC-framework that allows to take advantage of the back-drivability of a robot thus avoiding the use of force sensors. Then, as a casestudy, the iTaSC-framework is used to formulate a (positionposition) teleoperation scheme. The theoretical findings are experimentally validated using a PR2 robot.

A Pragmatic Method for Stable Stiffness Reflection in Telesurgery

At present, surgical master-slave systems lack any kind of force feedback. Typically, controllers giving good stiffness transparency for soft environments cannot guarantee stability during hard contact. This paper presents a pragmatic method to avoid instability of a master-slave system during hard contacts, which does not affect the stiffness reflection for soft environments. The time derivative of the interaction force with the environment is used to detect a hard contact. Upon detection of a hard contact the force feedback is switched off and a virtual wall is activated at the master side in order to guarantee the perception of hard contact by the operator. The experiments demonstrate good stiffness transparency for soft environments, while the system remains stable for both soft and hard environments.