Analysis and design of a cable-driven mechanism for a spherical surgery robot

This paper presents design and analysis of a cable-driven mechanism for a spherical laparoscopy surgical robot manufactured at the Research Center for Science and Technology in Medicine. The design is featured by two types of `idler pulleys´, which allow change of cables´ plane of motion in spherical workspace. This robot suffers from back-lash and high friction due to the usage of motor and gear-box in each joint. In order to find a precise and optimal design, the paper studies various designs of cable robots, considering pulleys and cable arrangements, as well as pre-tensioning methods. Final scheme introduces two series of pulleys for each DOF, which transmits power from motors located on base, to the corresponding joints. Each paired pulleys are in relation via two separate cables ending on both pulleys. First and second sets provide 227 and 313 reduction ratio in 3 and 4 stages respectively, resulting 95% and 93% efficiency, assuming 7 year life cycle. The stiffness is assessed by evaluating each paired pulleys´ stiffness and computing the whole series stiffness. Applying the maximum needed torques at the tip, through the entire workspace, we achieved the maximum tip´s displacement, 3.8 mm.