The modular interactive computer-assisted surgery (modiCAS) project, settled in the Center for Sensor System (ZESS) at the University of Siegen, in Germany, is engaged to develop an integral solution for different surgical problems by the combination of a navigation system and a robot arm with hands-on capabilities. The robotic system may be thought of as a smart surgical tool that extends surgeon's ability to treat patients, giving him/her surgical assistant by working in cooperative fashion. However, a natural and seamless integration of robotic systems in the operating room is still a big challenge in robotic surgery. The interaction between surgeon and robotic system is a very important issue. Autonomous systems have lost acceptance in the surgical community because the surgeon wants to be in charge of the operation rather than acting only as an observer. In such autonomous procedures, human experience, intuition, reacting capability in front of unexpected situations are lost. Furthermore, assistance intends to improve the performance of the surgeon rather than delimitating or obstructing him/her. An alternative solution is to provide a cooperative system where benefits of both can be combined. In this context, the surgeon gain complete control over the operation by grabbing the tool mounted on the robot and commanding it with his/her own hands. But the fact that a robot is to be used in clinical applications and in direct contact with human beings, imposes some additional requirements in comparison with the well established robotics technology applied in the industry. The most obvious is safety. On the one hand, the surgeon must keep control of the surgical operation. On the other hand, the surgical robots must assure a correctly usage by the surgeon in order to guarantee patient safeness. Therefore, surgeon's freedom of action has to be partially limited so that forbidden regions become unattainable to prevent accidental injuries. For these reasons, a seamless and safety integration of the system within the operating room is considered a paramount issue for a successful assistance and represents an important requirement within the modiCAS project.
The first contribution of this work is a proposal of software framework architecture for the modiCAS system able to support medical interventions in several surgical disciplines. A modular structure, together with a strategic distribution of the modules, provides flexibility for the adaptation of a common basic hardware platform to different applications.
Secondly, concerning the cooperative capabilities of the modiCAS system, the issue of mishandling is avoided with the introduction of virtual constraints, here called virtual fixtures, which help guiding the tool within certain predefined permitted directions. The combination of these preferred directions permits different kinds of virtual constraints, which may be one dimensional (lines), two dimensional (planes), three dimensions (tubes, cones, etc.) or even more complex trajectories (by means of parametric functions).
A particularly issue to be considered when thinking about cooperative manipulation of a robotic arm is the presence of singular configurations. In the neighborhood and at singular configurations an exact solution of the robot inverse kinematic becomes ill conditioned. Consequently, unfeasible joint velocities may be produced which yields into acute behavior of the robotic system. During virtual constrained cooperative operation, high velocities and position deviations are unacceptable. Therefore, another important objective of this work is to assure that such cooperative guidance is robust and accurate even in the presence of such singular robot configurations.