Measurement Principles of Optical Three-Axis Tactile Sensor and its Application to Robotic Fingers System

A tactile sensor is a device that can measure a given property of an object or contact event through physical contact between the sensor and the object. Traditionally, tactile sensors have been developed using measurements of strain produced in sensing materials that are detected using physical quantities such as electric resistance and capacity, magnetic intensity, voltage and light intensity (Nicholls, 1990). Research on tactile sensor is basically motivated by the tactile sensing system of the human skin. In humans, the skin’s structure provides a mechanism to simultaneously sense static and dynamic pressure with extremely high accuracy. Meanwhile in robotics, several tactile sensing principles are commonly used nowadays, such as capacitive, piezoelectrical, inductive, piezoresistive, and optoelectrical sensors (Schmidt et al., 2006, Lee & Nicholls, 1999). In our research lab, with the purpose to establish object manipulation ability in robotic fingers, we developed a hemispherical shaped optical three-axis tactile sensor capable of acquiring normal and shearing forces to mount on the fingertips of robot fingers. This tactile sensor uses an optical waveguide transduction method and applies image processing techniques. Such a sensing principle is expected to provide better sensing accuracy to realize contact phenomena by acquiring the three axial directions of the forces, so that normal and shearing forces can be measured simultaneously. This tactile sensor is designed in a hemispherical dome shape that consists of an array of sensing elements. This shape is to mimics the structure of human fingertips for easy compliance with various shapes of objects. For miniaturization of the tactile sensor, measurement devices are placed outside the sensor. The small size of the sensor makes it easy for installation at robotic fingers. The optical three-axis tactile sensor developed in this research is designed in hemispherical shape, and the sensing elements are distributed in 41-sub region. Due to this structure, the acquired images by CCD camera, except for sensing element at the sensor tip area, are not the actual image of contact pressure at the sensing elements. Therefore, to compensate with the sensor structure, it is necessary to conduct coordinate transformation calculations for each sensing element except for the element at the sensor tip area. In this chapter, we