Grip Force Control Based on the Degree of Slippage Using Optical Tactile Sensor

Recently, many kinds of tactile sensors for robot gripper have been proposed. One of the problems of these sensors for dexterous handling is to requiring information of grasping object such as the weight and the friction coefficient. They are difficult to be measured before grasping. In this paper, we propose a control method for robot gripper not using the friction coefficient. For this purpose, we use a vision-based tactile sensor which consists of a CCD camera, LED lights, acrylic plate and spherical elastic body. The feature of this sensor is measuring "stick ratio", which shows the degree of slippage between the robot gripper and the grasping object. When an elastic body contacts a rigid body and slides slightly, two regions, called "stick region" and "incipient slip region", appear in the contact area. The stick ratio is the ratio of the area of incipient slip region to that of the contact area. We propose the control system for robot gripper to achieve dexterous handling using the stick ratio. The stick ratio measured by the tactile sensor is used to control the grip strength. As a result, macroscopic slippage is prevented by control. It is experimentally showed that our control method for robot gripper makes it possible to achieve dexterous handling only using signals from the tactile sensor.