Fiber Bragg Gratings in CYTOP Fibers Embedded in a 3D-Printed Flexible Support for Assessment of Human–Robot Interaction Forces

Abstract

We developed a flexible support with embedded polymer optical fiber (POF) sensors for the assessment of human–robot interaction forces. The supports were fabricated with a three-dimensional (3D) printer, where an acrylonitrile butadiene styrene (ABS) rigid structure was used in the region of the support in which the exoskeleton was attached, whereas a thermoplastic polyurethane (TPU) flexible structure was printed in the region where the users placed their legs. In addition, fiber Bragg gratings (FBGs), inscribed in low-loss, cyclic, transparent, optical polymer (CYTOP) using the direct-write, plane-by-plane femtosecond laser inscription method, were embedded in the TPU structure. In this case, a 2-FBG array was embedded in two supports for human–robot interaction force assessment at two points on the users’ legs. Both FBG sensors were characterized with respect to temperature and force; additionally, the creep response of the polymer, where temperature influences the force sensitivity, was analyzed. Following the characterization, a compensation method for the creep and temperature influence was derived, showing relative errors below 4.5%. Such errors were lower than the ones obtained with similar sensors in previously published works. The instrumented support was attached to an exoskeleton for knee rehabilitation exercises, where the human–robot interaction forces were measured in flexion and extension cycles.

Polymer Optical Fiber-Based Integrated Instrumentation in a Robot-Assisted Rehabilitation Smart Environment: A Proof of Concept

“…Thus, the user forced the orthosis structure to angles below 0 • , and, despite the physical limits, the orthosis can show angles lower than 0 • due to backlashes in the components and can also lead to misalignments in the POF curvature sensors, leading to the measurement of angles below 0 • . Regarding the interaction forces, the results are similar to the ones measured in previous work for the same condition [18], where in the majority of the curves, it is possible to see two peaks: one due to flexion movement and the other for the extension movements. In the last flexion/extension cycles, there is also a decrease in the interaction forces, indicating fatigue of the user.…”

“…There are many optical fiber sensor technologies, which include sensors based on nonlinear effects [17], fiber Bragg gratings (FBGs) [18], interferometers [19] and intensity variation-based sensors [20]. Most of the aforementioned sensors need equipment such as an optical spectrum analyzer in order to acquire the sensor response.…”

Performance Analysis of a Lower Limb Multi Joint Angle Sensor Using CYTOP Fiber: Influence of Light Source Wavelength and Angular Velocity Compensation

“…Optical fiber sensors (OFS) present advantages such as being lightweight, compact, chemically stabile, immune to electromagnetic fields, and for their multiplexing capabilities [8]. In addition, these sensors are attractive option for wearable applications, especially for joint angle measurement, due to their lower dimensions and flexibility and ability to be embedded in clothing or flexible structures [9,10].…”

“…PMMA is the most common and popular material with a low cost [17]. However CYTOP has the lowest transmission loss among them [8,20], from the visible to near-infrared spectrum, which is a good option for sensing applications [9]. Moreover, CYTOP fibers present lower Young's Modulus, which means higher flexibility [21] and lower hysteresis, when used as sensor, than PMMA [22].…”

“…Since the polymers are viscoelastic materials, they do not have constant response to stress and strain, especially in dynamic loading conditions [22]. The polymer response under a constant stress or strain dependents on the time that the load is applied (or removed), which can result in different responses under different angular velocities and accelerations [9], and can restrict the application in a small velocity range. To overcome these limitations and expand the application for any angular velocity, compensation techniques are proposed, such as [7] in which a hysteresis compensation was developed based on curves relating hysteresis and the angular velocity of the sensor.…”