Embedded optical sensing for robots in extreme environments [electronic resource]
Keywords: Mechanical Engineering

Force sensing is an essential requirement for dexterous robot manipulation. Metal or semiconductor strain gages are commonly used for measuring forces. However, for certain uses in extreme environments, such as extra-vehicular activities in space and magnetic resonance imaging (MRI)-guided robotic surgeries, optical fiber Bragg grating (FBG) sensors have compelling advantages: they are immune to electromagnetic noise, physically robust (especially when embedded in solid parts), and able to resolve very small strains. In addition, with optical multiplexing, many sensors can be located along a single fiber and interrogated in parallel. This thesis first describes composite robot end-effectors that incorporate optical fibers for accurate force sensing and control and for estimating contact locations. The overall design is inspired by biological mechanoreceptors, such as slit sensillae or campaniform sensillae, in arthropod exoskeletons that allow them to sense contacts and loads on their limbs by detecting strains caused by structural deformation. A new fabrication process is presented to create multi-material reinforced robotic structures with embedded fibers. The results of experiments are presented for characterizing the sensors and controlling contact forces in a closed loop system involving a large industrial robot and a two fingered dexterous hand. The proposed exoskeleton finger structure was able to detect less than 0.02 N of contact force changes and to measure less than 0.15 N of contact forces practically. A brief description on the optical interrogation method, used for measuring multiple sensors along a single fiber at kHz rates needed for closed-loop force control, is also provided. Following the successful creation of force sensing robot fingers in a large-scale (120 mm long) prototype, the finger structure was miniaturized to a human fingertip scale (15 mm long) for robots designed for human interactions in space. For miniaturization, a bend-insensitive optical fiber was selected to be embedded in a small fingertip. The small fingertip was able to measure 3-axial forces with practical force measurement resolutions of 0.05 N for forces applied transverse to the finger and 0.16 N for forces applied axially to the fingertip. As an extension of the application of FBG sensors to robotic devices used in extreme environments, a magnetic resonance imaging (MRI)-compatible biopsy needle is instrumented with FBGs for measuring bending deflections as it is inserted into tissues. During procedures such as diagnostic biopsies and localized treatments, it is useful to track any tool deviation from the planned trajectory to minimize positioning error and procedural complications. The goal is to display tool deflections in real-time, with greater bandwidth and accuracy than when viewing the tool in MR images. A standard 18 ga (approximately 1 mm diameter) x 15 cm inner needle is prepared using a custom-designed fixture, and 350 um deep grooves are created along its length. Optical fibers are embedded in the grooves. Two sets of sensors, located at different points along the needle, provide a measurement of an estimate of the bent profile, as well as temperature compensation. After calibration, the measured tip position was accurate to within 0.11 mm. Tests of the needle in a canine prostate showed that it produced no adverse imaging artifacts when used with the MR scanner and no sensor signal degradation from the strong magnetic field.