Compared with traditionallaparoscopic surgery, robot-assisted surgery can return all six degrees of freedom (DOF) to the surgeon, provide stereovision, filter tremors and increase precision for positioning and manoeuvring the surgical instruments. The instruments used with commercially available surgical robots use external actuation in the form of relatively large motors located outside the patient with a cable transmission system to bring rotation to the instrument's wrist. The goal of this research is to shift away from the external actuation and design a surgical instrument that is internally actuated. This is expected to miniaturize the overall device, enable instruments to be created with a greater number of DOF than possible using the cable-driven approach, enable the creation of modular designs with a "plug and play" capability and increase the precision ofposition and force control.
A novel internally-actuated instrument has been designed and prototyped using 6 mm DC motors and miniature transmissions. It features four DOF: an elbow joint, a roll joint, and a wrist joint that employs two independently-actuated gripper jaws to allow for both rotation and grasping ability. The elbow joint is a unique feature that helps to avoid collisions with internal organs. The design of the instrument has been explored in detail. After outlining the target specifications of the device, justification is provided for the selection of the DC motors. Additionally, the thermal properties of the motors have been examined to determine safe current limits.
The design of the transmission mechanism (lead screw plus slider-crank) has been analysed and an optimization algorithm for the slider-crank parameters has been developed. Design calculations have been conducted to analyse the kinematics and static loading of the device and finite element analysis (FEA) has been executed to determine the stress concentrations due to the loading. Justification is also given for the component and material selection.
A prototype intended as a kinematic model has been manufactured and assembled. The speed performance of the prototype has been tested using two methods: the first used video motion analysis to determine the average speeds of the elbow, roll and wrist joints; the second utilized a potentiometer to measure the instantaneous speed profile across the range of elbow joint motion. Overall, the elbow joint operated at an average speed of 2.0 rpm, the roll joint operated at 40 rpm, and the gripper jaws in the wrist operated at around 3.8 rpm. The potentiometer tests revealed that the joint performed in accordance with the theoretical speed profile, particularly when a correction factor was applied to account for the actual current that was drawn by the motor.
A force experiment was also conducted to confirm the torque capabilities of the miniature brushed DC motor used in the prototype. Results showed that the motor, attached to the lead screw and slider components of the elbow joint mechanism, performed at about 15% efficiency. The motor was able to supply a torque of up to 4.2 mNm while maintaining an acceptable current level to avoid over-heating.