Preliminary Tests of the Miniaturization of a Novel Concept of Angular Sensors

This work was done in the framework of the MIRACLE project devoted to the development of a flexible, articulated robotic endoscope to perform laser osteotomies (bone cutting). In order to control this robot with the required accuracy of 0.2 mm at the tip position, a tracking system needs to feed back the pose of the tip and the shape of the endoscope. One of the possible designs of such a robotic endoscope is based on an articulated device that can be tracked by measuring the bending angle at each joint. Recently, we presented (Iafolla et al., 2018) a novel angular sensor called ASTRAS (Angular Sensor for TRAcking Systems), which measures a one degree of freedom rotation by processing a shadow image cast through a shadow mask onto an image sensor. One ASTRAS will be placed at each joint of the articulated robotic endoscope in order to measure their bending angles. The planned diameter of the MIRACLE endoscope is 10 mm and it will contain several components such as the optical fiber for the laser, irrigation and sucking systems, and robot control mechanisms. However, the latest version of ASTRAS covered a volume of 40x40x55 mm(3). For this reason, the most demanding requirement for the design of the final ASTRAS sensor is its miniaturization to fit the sensor inside the joint of the endoscope. Consequently, in this contribution, we investigated the possibility of miniaturizing ASTRAS and assess its limitations due to some fundamental aspects. In particular, we built a prototype based on a NanEye camera by AMS (1x1x0.5 mm(3)) and a micro-milled shadow mask with 0.08 mm wide slits. The tests of this prototype showed encouraging results and will be discussed. In particular, at such small dimensions, there are no unexpected optical effects (e.g. optical diffraction due to the mask) worsening the measurement. Indeed, the precision of 5 arcsec and the linearity error of 23 arcsec RMS are comparable to those of the previous bigger prototype. Further considerations and concepts related to the miniaturization of ASTRAS are also discussed in this paper.