Fig. 1. Overview of the OFN sensor. (a) Schematic diagram of the OFN sensing system. (b) 5 by 5 OFN sensing array. (c) Photographs of the output light when the sensor was untouched, (d) subjected to normal force, and (e) subjected to both normal and frictional force. (f) Finite element simulations of strain distribution when the OFN sensor (knot diameter: 3.5 mm, side length: 5 mm, thickness: 1 mm) was untouched, (g) subjected to a 10 N normal force, and (h) subjected to a 10 N normal force and an additional 4 N frictional force, respectively.

Fig. 2. Characterization of normal and frictional force sensing. (a) Schematic diagram of the friction testing system. (b) Responses to normal force of OFN sensors with knot diameters of 4.5 mm, 3.5 mm and 2.5 mm. (c) Responses to frictional force of OFN sensors with a diameter of 2.5 mm when pre-loaded with 0 N, 2.5 N, 5 N, 7.5 N and 10 N normal forces. (d) Intensity signals of the OFN sensor loaded/unloaded with normal and frictional forces. (e, f) Intensity signals during cyclical loading of normal force and frictional force, respectively.

Fig. 3. Adaptive grasping based on slip detection when the slip feedback is ON and OFF, respectively. (a, e) Snapshots during the experiments. Yellow arrows indicate slips of the cup while green arrows indicate closing of the robotic hand. (b, f) Real-time signals of the OFN sensors. (c, g) Real-time scores calculated by the Slip Finder program. Slip Score = 1: a slip occurred, Slip Score = 0: no slips. (d, h) Real-time position of the robotic fingers. Position = 0: fully open, Position = 255: fully closed.

Fig. 4. Characterization of tri-axial force sensing. (a) Schematic diagram of the tri-axial force testing system. (b–d) Stress distribution and responses of OFN sensors with knot diameters of 4.5 mm, 3.5 mm, and 2.5 mm to F_x, F_y, and F_z, respectively.

Fig. 5. Dexterous manipulation based on tri-axial force sensing. (a) Exploded diagram of the robotic tactile finger. (b) Positions of the five OFN sensors inside the finger. (c) Schematic diagram of force decomposition. (d) Photograph of two robotic tactile fingers. (e, f) The tri-axial force sensing signals and snapshots during the cutting experiment and the unlocking experiment, respectively.