Design of a multi-DOF cable-driven mechanism of a miniature serial manipulator for robot-assisted minimally invasive surgery* Antonia Tzemanaki 1 , Lukasz Fracczak 2 , David Gillatt 3 , Anthony Koupparis 3 , Chris Melhuish 1 , Raj Persad 3 , Edward Rowe 3 , Anthony G. Pipe 1 , and Sanja Dogramadzi 1 Abstract— While multi-fingered robotic hands have been de- veloped for decades, none has been used for surgical operations. μAngelo is an anthropomorphic master-slave system for tele- operated robot-assisted surgery. As part of this system, this paper focuses on its slave instrument, a miniature three-digit hand. The design of the mechanism of such a manipulator poses a challenge due to the required miniaturization and the many active degrees of freedom. As the instrument has a human- centered design, its relation to the human hand is discussed. Two ways of routing its cable-driven mechanism are investigated and the method of deriving the input-output functions that drive the mechanism is presented. I. INTRODUCTION Technical surgical competence, such as manual dexterity, is a basic and very important component for a surgeon [1]. Surgical robots can offer improved precision and dexterity [2], especially when the manipulation of the surgical instru- ments and control of the surgical robot comes naturally to the surgeon. We have previously presented the concept of anthropomor- phic surgical instruments in order to help reduce the ‘cogni- tive gap’ between the current manipulation of robot-assisted minimally invasive surgery (RAMIS) instruments and the surgeon’s natural hand movements [3]. The μAngelo sys- tem for RAMIS aims at higher dexterity and precision required for surgical tasks and shortened training time for new surgeons. It utilizes an anthropomorphic approach: the surgeon controls hand-like instruments by wearing a lightweight sensory exoskeleton (Fig. 1) [4]. The instrument has 13 actuated degrees of freedom (DOF) in total and accommodates a cable-driven mechanism inside the digits and through the shaft. Tendon driven mechanisms impose coupling of the joints’ motion and complicate their actuation and control, while there is also the risk of tendons breaking during an operation. Other methods include the use of shape memory alloy [3], al- though grasping forces seem insufficient for use in abdominal surgery. Hong et al. use a parallel rigid link mechanism with the drive unit integrated into the instrument’s shaft [5]. MICA also has its motors integrated into the instrument in order to aim for versatility and low cost as the tool is detachable [6]. *This work has been supported by the Bristol Urological Institute antonia.tzemanaki@brl.ac.uk 1 Bristol Robotics Laboratory, Frenchay Campus, University of the West of England, Bristol, UK 2 Institute of Machine Tools and Production Engineering, Lodz University of Technology, Poland 3 Bristol Urological Institute, Southmead Hospital, Bristol, UK (a) (b) Fig. 1. μAngelo surgical system: (a) 3-digit surgical instrument and (b) sensory hand exoskeleton for remote control The literature shows that there is a compromise between the size of a manipulator and the number of its independently actuated DOF. For example, MICA’s end-effector (universal joint) is controlled via a cable-pulley system via the 10 mm diameter shaft. Both the Da Vinci [7] and the Raven instruments [8] use a cable and pulley system as well. The shaft of Da Vinci instruments is 8 mm in diameter, with a few exceptions at 5 mm. The Robin Heart system for cardiac surgery has a tool with changeable tooltips and 3 DOF [9]. In a non-surgical context, the DLR Hand has fingers that have 4 actuated DOF using an antagonistic pair of tendons for each DOF and a total of 8 motors, while its size is comparable to a human hand [10]. Table I compares the size and active DOF of robotic digits (of surgical instruments or hand). Note that opening and closing of functional end-effector has not been included in the number of DOF. In this paper, we focus on the cable-driven mechanism of the μAngelo instrument. Although it resembles a miniature hand, we believe that exact imitation would be unnecessary for RAMIS applications. Therefore, the ‘anthropomorphic’ concept is compared to the human hand, demonstrating their similarities as well as their differences such as the relation of the human hand tendons and the instrument’s mechanism. TABLE I COMPARISON OF CABLE- DRIVEN MECHANISMS IN ROBOTIC INSTRUMENTS OR FINGERS System IAD D (mm) Da Vinci 3 5 Raven 3 10 MICA 2 10 Arata et al. 2 10 RobIn Heart 3 10 DLR Hand (finger) 4 n/a** μAngelo (thumb) 5 6 *IAD: No. of indepedently actuated DOF, D: outer diameter of the shaft ** similar to a human finger