A Cable-Driven Grasper With Decoupled Motion and Forces1

摘要

Minimally invasive surgery is an operation that uses specially designed surgical tools inserted through small incisions to work on human tissue. Due to its distinct advantages such as minimizing incision size, shortening recovery time, and lowering infection risk, it has been widely implemented in hospitals around the world. However, because of friction and the long transmission distance between the grasper tip and tool handle, the surgeon cannot readily perceive the tactile information, which may adversely affect surgical efficiency and/or efficacy.One method to address this issue is to incorporate force sensors on the jaws to measure clamp force [1,2] (Fig. 1), but this makes the grasper tip bulky and the grasping potentially less reliable. Another way to solve this problem is to attach a sensor on the distal end of the shaft near the grasper tip [3,4] (Fig. 2), but this measures net applied force as opposed to clamp force, which is more directly relevant to tool-tissue interactions. Moreover, most sensors cannot survive the harsh environments needed for sterilization of instruments.The general idea of this research is try to accurately sense the clamp force indirectly, with or without sensors. A decoupled grasper design based on planetary gear theory has been proposed [5], which makes possible a direct sensing approach using the actuators. This paper builds on the decoupled design to present the prototype and the decoupling experiments relevant to grasp force measurement.A scaled prototype was 3D printed based on the design in Ref. [5]. All the joints in the grasper tip are equipped with ball bearings to reduce friction. Monofilament fishing line (nylon or polyvinylidene fluoride) is used for the cable transmission (Fig. 3). Two jaws are each driven independently via separate links attached to the cables, and the yaw motion is also driven by a pulley through a cable (Fig. 4).A force-sensitive resistor (FlexiForce A201 4.4N force range) is used to measure the clamp force; to make sure the force is equally distributed on the sensor, an intermediate pad is connected to one jaw using a spherical joint (Fig. 5). The clamp force measurement setup is shown in Fig. 6. To measure the cable force, a strain gage (Vishay MM WK-13-250AE-10C) is attached in series with one of the cables driving the jaws (Fig. 7).Three experiments have been done on the prototype. The first of these is to validate the motion decoupling, which means the grasp motion is completely independent of the yaw motion; the second experiment is to test the linear relation between clamp force and cable tension; the third is to test the force decoupling, which is similar to the first test but viewed from the perspective of force measurement as opposed to kinematics. For the force decoupling experiment, a protractor is used to measure the yaw angle (Fig. 8).The compliance of the monofilament nylon used for cable transmission introduced two effects that needed to be controlled: sufficient clamp force cannot be obtained on the jaws through the handle, which has a limited range of motion; and the accuracy of position measurements was affected. In the experiments, the cable is pulled by weights rather than driven by the handle, to improve the maximum clamp force, and additional constraint was added to hold the jaw in position to isolate kinematic effects of cable stretch.For the first experiment, the motion of the prototype follows the decoupling theory in Ref. [5], and matches the predicted motion of the computer-aided design (CAD) model (the yaw output angle is double the input, see Fig. 9). Furthermore, the position of the two jaws remains constant independent of yaw angle. This experiment proves that the grasp motion is decoupled from the yaw motion.For the second experiment, with different tensions in the cable, the corresponding clamp force was measured. The result is shown in Fig. 10, in which the linear relation between cable tension and clamp force is clearly shown