This work reports the development of a bioinspired sensor capable of measuring vertical and shear (tangential) forces. The sensor is composed of two materials, the polylactide (PLA) and epoxy resin, combined with a photosensitive optical fiber with two fiber Bragg gratings (FBG1 and FBG2). The FBG1 was placed in a cavity filled with epoxy resin, while FBG2 was between the cavity and the shear wall that undergoes shear force. This FBGs’ encapsulation allowed one of them to be affected by vertical and shear forces (FBG1), while FBG2 was only affected by shear force. The calibration and performance tests were carried out with the aid of an electronic tri-axial force sensor. From these tests, sensitivities of K1V= 0.02±2.35x10-4 nm/N; K1S= 0.13±3.25x10-3 nm/N; K2V= -2.88x10-4±6.72x10-5 nm/N and K2S= -1.77±0.03 nm/N to each type of force, for FBG1 and FBG2, respectively, were achieved. The obtained results demonstrated the reliability of the developed solution, with a significant improvement of its sensitivity to shear force, and a low production complexity, when compared to other previously reported optical sensors.
This work consists on the design and implementation of a compact and accurate biaxial optical fiber sensor (OFS) based on two in-line fiber Bragg gratings (FBGs) for the simultaneous measurement of shear and vertical forces. The two FBGs were inscribed in the same optical fiber and placed individually in two adjacent cavities. In the calibration and performance tests, the response from the optical fiber cells was compared with the values given by a three-axial electronic force sensor. Sensitivity values obtained for the FBG1 are K1V= (14.15±0.10) pm/N (vertical force) and K1S= (-26.02±0.08) pm/N (shear force) and for the FBG2 are K2V= (7.35±0.02) pm/N and K2S= (-24.29±0.08) pm/N. The conversion of the Bragg wavelength shift, given by the optical fiber sensors, into the shear and vertical force values is also presented along with its comparison to the values retrieved by an electronic sensor, yielding to low RMSE values, which shows the high accuracy of the algorithm applied. This work stands out from the others with optical fiber by the simplicity of its structure. The proposed solution represents a compact and reliable device for simultaneous measurement of shear and vertical forces, useful in several areas, such as: incorporation into insoles for plantar pressure and shear force measurement; electronic skin technologies; smart rehabilitation robotic exoskeletons; or even biomimetic prosthesis.
In an era of unprecedented progress in technology and increase in population age, continuous and close monitoring of elder citizens and patients is becoming more of a necessity than a luxury. Contributing toward this field and enhancing the life quality of elder citizens and patients with disabilities, this work presents the design and implementation of a noninvasive platform and insole fiber Bragg grating sensors network to monitor the vertical ground reaction forces distribution induced in the foot plantar surface during gait and body center of mass displacements. The acquired measurements are a reliable indication of the accuracy and consistency of the proposed solution in monitoring and mapping the vertical forces active on the foot plantar sole, with a sensitivity up to 11.06 pm/N. The acquired measurements can be used to infer the foot structure and health condition, in addition to anomalies related to spine function and other pathologies (e.g., related to diabetes); also its application in rehabilitation robotics field can dramatically reduce the computational burden of exoskeletons’ control strategy. The proposed technology has the advantages of optical fiber sensing (robustness, noninvasiveness, accuracy, and electromagnetic insensitivity) to surpass all drawbacks verified in traditionally used sensing systems (fragility, instability, and inconsistent feedback).