Spherical wrist dimensional synthesis adapted for tool-guidance medical robots

The objective of this article is to present the dimensional synthesis of serial and parallel spherical wrists, an important step in the design process of medical robots. This step is carried out to obtain optimal dimensions of tool-guidance medical robots. With this goal, we have first studied the specifications of two robots with different medical applications: one for tele-echography examination and one for minimally invasive surgery. Then, we have established that the medical needs expressed by the doctors were very different but the specifications in robotic terms have a lot of common points (kinematics, workspace, bulkiness). For both applications studied, robots need a mobility of three rotations around a fixed point (probe contact point on the patient's skin or trocar incision). So, a spherical wrist architecture is adapted to their needs. An important constraint related to medical applications is that the robot must be compact in order to not obstruct or collide with its environment (medical personnel or patient). We perform dimensional synthesis allowing determination of dimensions of the mechanism for serial and parallel spherical wrists, for a tele-echography robot, and a serial wrist for a minimally invasive surgery robot. We use multi-criteria optimization methods minimizing a cost function to obtain both good kinematic performance and compactness for the architecture. The difficulty/challenge of this design process, depending of the studied applications, is the choice of efficient criteria describing the performances and the constraints of the robot. The design variables must faithfully represent the specifications of the robot so that its performance can respond to the medical requirements. We show, here, the different methods used for optimizing the chosen kinematic architecture for the particular medical application. These studies lead to prototypes which are validated after medical experiments. This process of dimensional synthesis will be applied to other medical applications with different sets of specified constraints.