Haptic feedback method using indirect measurement for surgical robots

Abstract

Surgical robot systems have been rapidly developed and attracted considerable attention in recent years. The basic concept is to perform a surgical task directly using a master-slave robot system controlled by the surgeon at the remote-site. A MIS (Minimally Invasive Surgery) which is part of the surgical robot is less invasive than open surgery, and is performed through the skin or through a body cavity or anatomical opening. This may result in reduced duration of hospital stays, blood loss, transfusions, and less pain. The surgical robot is controlled by a surgeon at a distance from the patient. Surgical tasks are conducted by a robotic gripper system attached to the slave surgical robot. The surgeon depends only on the laparoscope CCD camera for visual feedback during the operation in surgical robot systems. The surgeon depends on the haptic palpation to distinguish the internal organs from bones and recognize ruptures during the operation in practical surgery. Sometimes the medical malpractice can be occurred by the dead zone of vision during surgery. Therefore, several researchers focused on the environment force measurement for realizing the force feedback. There are three category in these force measurement, i.e., the direct measurement at the tip, the position difference based force estimation method, and sensor based force estimation method. Of these, acquisition of the environment force at the instrument tip can enable better performance, e.g., transparency of the haptic feedback in the surgical robot systems as compared to the position difference based force estimation method. It is, however, difficult to measure the environment force directly due to technical limitations in attaching sensors to the tip of the instruments. This dissertation proposes a method to estimate the forces by installing the sensors away from the instrument tip. The transmitted process of the environment forces are analyzed to determine the specific mechanical parts of slave robot arm. The proposed method employs specially designed mechanical parts of the slave robot, i.e., the top brackets for the z-axis translational force along the insertion direction, and the docking clamps for the rotational pivot torques around the fulcrum point. In addition, the proposed method can recover the environment force as minimizing the change of conventional design without any additional device or system. Another advantage of this design can completely avoid the contamination of sensor systems during the replacement of the contaminated surgical instrument, which is performed several times during surgery. Strain gauges are attached to specially designed places with enhancing shapes. The simulation results of the force estimation are presented to confirm the strain concentration area. The proposed method is validated with quantitative experimental results. Calibrated weights are decided upon the comparison of the strain value with a calibrated 6-axis Force/Torque (F/T) sensor. There are coupled between xy and z-directional force. Although the z-directional force does not affect to the docking clamps, the x and y-directional forces affect to the top brackets. It can be solved by the sensor calibration process by using the linear transformation matrix between the strain gauge sensor signals and the force vector. The RMSE of force sensing performance are about 5 ~ 7 %. These errors are also the permissible force sensing error as below 10 % of JND. It means a human cannot discriminate the force difference when the force sensing error is within 10 %. A multi-DOF device modeling is necessary to analyze the kinematics and Jacobian of the master and the slave device for calculating the force decomposition from the measured environment force. The communication channel between the master and the slave is a UDP connection for transferring the position and the force information. Additionally, a 6-axis F/T sensor is attached at the end effector of the master device for measuring the exerted operator force. The tracking performance of master-slave robot system is experimentally conducted. Finally, the haptic feedback experiment is carried out for verifying the transmitted environment forces at the master device. The direct force feedback which is a part of a FP control architecture is used for constructing the master-slave robot system. Accurate direction of reflecting forces are verified according to the kinematic difference between multi-DOF master and slave device. The measured environment stiffness errors at slave site are about 3-10 % in translational and rotational motion, and the measured transmitted stiffness errors at master site are about 5-11 % in translational and rotational motion. The variation error of the measured stiffness can be estimated about $\pm 60 N/m$ in translational motion. It is because the strain at the top bracket can be changed due to the change of the line contact while the slider motion. Although the force sensing errors are existed, the human can discriminate these phantom tissues depend on the different transmitted stiffness. It means that the proposed method has an enough force sensing performance for discrimination.