Trans-barrier local magnetic actuation for robotics abdominal surgery

© 2018 Dr. Florence Ching Ying LeongThe advances in surgical instruments have played a significant role in the reduction of surgical trauma on patients. The evolution of surgical applications to the current popular approaches of minimally invasive surgery (MIS), laparoendoscopic single-site surgery (LESS) and natural orifice transluminal endoscopic surgery (NOTES) integrated with the use of magnetic actuation emphasizes the benefits of magnetic systems in the field of abdominal surgery. The realisation of the magnetic-based surgical approaches has been explored through the use of magnets or magnetic sources (e.g. electromagnets) by exploiting magnetic linkages transmitted across the abdominal wall to guide, anchor and actuate surgical devices intra-abdominally, which introduces the local magnetic actuation (LMA) technique. A LMA unit within the system consists of an external actuating magnetic source driving a corresponding permanent magnet rotor in the internal surgical device to drive or actuate a degree-of-freedom (DOF) on a robotic manipulator. As the magnetic transmission from the external driving unit is closely targeting the internal driven units, hence localised, multiple LMA units can be used to drive multiple DOFs on a multi-DOF robotic surgical manipulator within the abdominal workspace. The use of permanent magnets as actuation sources to drive the LMA system was previously tested. Nonetheless, having fixed parameter (i.e. magnetic field), it is challenging for the magnets to adapt to configuration changes in the surgical setting, such as the variations in abdominal wall thickness as well as the misalignment between the external driving and the internal driven units. These variables, if not compensated, would affect the transmission of torque across the abdominal wall onto the permanent magnet rotor in the internal surgical device as magnetic field transmission reduces exponentially against distance. Hence, in this study, electromagnetic stators are used in place of the actuating permanent magnets, coining the term Local Electromagnetic Actuation (LEMA), to provide control variables (i.e. varying current signals to the stators) to compensate for the changes in transmitted torque due to the variables. Although the LEMA approach addresses the above limitation faced by the permanent magnet version of the LMA system, there are nonetheless other challenges that need to be dealt with for the LEMA system to work effectively. While there are many factors contributing to these challenges, two major problems inherent to the design and configuration of the LEMA system are identified to be addressed in this thesis. These crucial problems are due to the systematic disturbances within the system, and if not properly handled, will affect the performance of the system. One of these disturbances onto the LEMA system is caused by the magnetic interaction or interference by the neigbouring magnetic source(s) within a multi-DOF configuration. This is due to the close proximity the magnetic sources has to be placed to one another in a limited workspace above and within the abdominal cavity. This magnetic interference impacts on the performance of the system by causing irregularities in the angular velocity of the rotors, hence resulting in abrupt and inconsistent motions in the surgical device. Apart from the disturbance caused by magnetic interference, the internal surgical device also experiences disturbance due to the dynamics of the abdominal wall tissue as the device is anchored to the abdominal wall for surgical manipulation. This factor is often ignored as the tissue is assumed sufficiently rigid upon insufflation and magnetic anchoring. Nonetheless, the abdominal wall is a soft tissue of viscoelastic nature and is bound to encounter oscillations due to the attraction and repulsion forces caused by the external magnetic sources during magnetic actuation. Both the disturbances due to the magnetic interference and abdominal wall dynamics are caused by the actuation signals to the external electromagnets. These actuation signals are known to the system, hence the disturbances are referred to as systematic disturbances. This thesis reports the thorough investigations and analyses performed on the two major systematic disturbances onto the system, as well as the development of a controller to suppress these disturbances. In this thesis, a novel model of the LEMA system taking into account the magnetic field and the equation-of-motion at the point of each rotor has also been established and experimentally validated. The models of the aforementioned disturbances are constructed based on the information obtained from the disturbance investigations. The availability of the system and disturbances models allows the consideration of a model-based controller, hence the Internal Model Principle (IMP) controller design is implemented. The IMP method enables the models of the disturbances to be augmented into the model of the system for the design of a disturbance observer. The effectiveness of the IMP controller is numerically verified in simulations prior to experimental implementation. An experiment setup with two LEMA units, representing a multi-DOF LEMA system configuration, is constructed for performance validation of the IMP controller. The simulation and experiment results show effective rejection of both disturbances onto the system, through the observation on the speed response of corresponding rotors, hence demonstrating the accuracy of the system model and the efficacy of the controller design. The findings in this research serve to remove a significant technical hurdle in bringing this emerging technique of local magnetic actuation into practical reality for abdominal surgery