Optimal Design of a Novel Spherical Scissor Linkage Remote Center of Motion Mechanism for Medical Robotics

In this paper, a new remote center of motion (RCM) mechanism is presented whose end-effector is able to move through an entire hemisphere. In general, minimally invasive surgery (MIS) applications, an elliptic cone workspace with vertex angles of 60 degrees and 90 degrees gives the surgeon enough freedom to operate. Therefore, the majority of the developed RCM mechanisms have such a cone as the workspace. However, there are still situations in which a larger workspace is required, like the breast ultrasound scanning application in which the RCM mechanisms should be able to move over a hemisphere to do the breast scanning. The proposed RCM mechanismis developed based upon a spherical scissor linkage and benefits from the high stiffness characteristics of parallel structures while eliminating the common problem of linkage collision in parallel structures. It has two rotational degrees of freedom that are decoupled from each other. The Jacobian and the stiffness of the mechanism while considering the bending of the links is calculated through the virtual joints method (VJM). The kinemato-static equations and the methodology for calculating stiffness are described in detail. The optimal arc angle of the mechanism's links is found using a multi-objective genetic algorithm optimization. A prototype of the mechanism is built and forward kinematic of the proposed mechanism is examined experimentally. The experiments indicate that the proposed mechanism is able to provide a hemisphere as its workspace while the RCM point of the mechanism is fixed in the space.