Static Calibration of a Force sensor based on Fiber Bragg Technology

Since 1960 fiber optic sensors began to be employed due to several advantages: they are very sensitive, immune to electromagnetic interference and physically flexible, despite being rather bulky and a loss of light occurring when there is any micro bend in the fiber. They began to be applied to different fields thanks to their advantages. The power of optical fiber was first discovered and used in telecommunications, due to its fast response and the possibility to be multiplexing in order to collect more information. These sensors were also employed in medical fields and finally in robotics, in particular tactile sensing technologies.
Several kinds of tactile sensors have been invented using different devices: the resistive, the capacitive, the piezoresistive, the piezoelectric, the magnetic and the fiber optic sensors. For this scope, FOSs have the best characteristics, that could be best suited to the application required (for example lightweight less than 1 g, low power consumption less than 1 mW, thin in depth less than 1 mm, temperature range between -20°C to 60°C ). This new world is very little explored, principally due to the high cost of the sensors. Several typologies of FOSs exist, according to their physical principle of working. Moreover, they are divided in intrinsic and extrinsic sensors: the first ones are characterized by being the sensing element themselves, like the micro-bending, where the fibers are the transducers of the external effects through the internal bends. The second ones represent the tool, through which the information is carried up to the detection system that is the transducer of the environmental effects, like Fiber Bragg Grating (FBG), interferometry-based or intensity-modulated sensor.
In recent years, numerous researchers have investigated innovative methods to realize tactile sensing elements, using FOSs. Several reviews present works actualized with FBG, as in all the optical fiber sensors they are the most advantageous types: they are tiny, easy to multiplex and very sensitive with a fast response and no hysteresis. Moreover, the repeated experiments that were conducted showed that the quality of the performance were found to be excellent, well within the parameters of the human applications considered.
The tactile force sensor proposed is a 3x3 array of nine fiber optical sensors, made of a rigid rectangular structure of acrylic resin, covered by a film of Polydimethylsiloxane. For each row three optical fibers are glued together, so that there are three FBGs in series, with different Bragg wavelengths. In order to reproduce a static calibration, several measurements are conducted, applying a force on different points of the sensors: the force range is between 0-10 N with a speed of 0.1 mm/s. In particular the set of three tests was performed to investigate the repeatability of the experiments. The experimental data is detected with a LabView program on the computer and then elaborated in the MatLab environment: the final results are represented with graphs, where the Bragg wavelength shift of each sensor is in function by the applied force, and completed tables, that report the sensitivity and the uncertainty values; the results show that in almost all the tests the susceptibility of each sensor was not under 0.0015 nm/N, even considering the sensors that the force was not directly applied to. The uncertainty values are narrow enough, with a relative uncertainty of about 15% , if the highest values are considered. However, the force resolution of each fiber is at least 10 mN, while the spatial resolution is about 10 mm2.
Finally, these results show that the behavior of the sensor is adequate for the scope of the thesis, because the parameters of the developed tactile sensor comply with the requirements of the human application. This thesis wants to accentuate the main merits and demerits of the sensor, in order to discuss the future progress that could be carried out.