Abstract:

Several studies have shown that mechanism with Remote Center of Motion (RCM) are very useful for some medical applications, such as Minimally Invasive Surgery (MIS) or tele-echography. In MIS, surgical instruments must be rotated around an incision to navigate inside the patient. In tele-echography, an ultrasound probe is rotated around its tip which is in contact with the patient’s skin. In both cases however, it is necessary to control the position of the Center of Rotation (CoR) of the manipulated object (instruments, devices, etc.), either during the process, or as a preliminary adjustment. But most RCM mechanism are unable to reposition the CoR of their end effector. A simple solution to this issue is to mount them on a platform that provides linear motions. However, this conceptual design suffers from a significant increase in volume and weight.

By the mean of reconfigurable mechanisms, the present project suggests a more compact, lighter and possibly cheaper alternative. In this regard, a new concept of RCM mechanism with controllable CoR is proposed. It is intended to study and to design a series of RCM manipulators able to reposition their end effector CoR by themselves, thus suppressing the need for an additional linear platform. This present concept is based on RCM made of spherical linkages. This type of architecture is known for constraining its CoR by the intersection of their revolute joint axes. A reconfigurable spherical linkage has been specifically imagined to change its radius while maintaining its joint axes, in order to reposition its CoR. 

Based on the conceptual design of this reconfigurable linkage, two robotic prototypes will be created. One will be mounted as two reconfiguration linkages in series to form a serial spherical linkage. The second one will be a combination of four linkages, mounted as two serial spherical arms in parallel. In both cases, a two-DoF RCM will be generated with the ability of CoR repositioning. Their kinematic analysis will be performed and a model will provide the kinematic of their CoR. It is expected for the first prototype that the CoR reposition will not affect the RCM kinematics, which means that angular and linear motions will be decoupled. For the second prototype, the CoR repositioning is expected to have an impact on the RCM kinematics that will be studied. Each mechanism will be designed and manufactured in a robotic manipulator for testing. A series of motion capture experiments will allow validating the different derived models.