FBG-based sensing system to improve tactile sensitivity of robotic manipulators working in unstructured environments

The emergence of Industry 4.0 has brought new concepts to the factories that optimize and improve conventional processes. These technologies have brought assignments to the industrial robots that allow them to perform tasks faster and more precisely. The improvement of the robot’s proprioception capacity and tactile sensitivity using sensors is a useful approach to achieve those goals. Optical fibers are a viable technology to be used as sensors in robotic devices because they are electrically passive and present electromagnetic immunity. This paper proposes a Fiber Bragg Grating (FBG) based sensing system to monitor robotic manipulators during their operation. It corresponds to smart textiles installed on the robot’s body to detect interactions with the environment. A mathematical model is proposed to find what should be the greatest distance between adjacent FBGs to detect contact at any point between them. From this, it is possible to obtain a minimum number of sensors to detect contact at any point and guarantee the highest spatial resolution of the system with lower costs. The tactile system is formed of a group of optical fibers with multiplexed FBGs embedded in silicone rubber. The optical fibers with the sensors are positioned between layers of polyethylene foam and cotton fabric. After the manufacturing process, temperature and force characterization were done on the sensors which make up the smart textiles. In the characterization results, almost all the FBG presented values of R² on the linear regression superior to 0.94. Individual analysis is performed for the sensors which present a low coefficient of determination. Finally, the system was tested in an experimental validation in which the robot was hit while executing a task. From the results, it can be observed that the system can provide the position on the robot’s body, the amplitude in terms of force and the instant of time in which an external impact occurred.publishe

Low-cost Fiberoptic Probe for Ammonia Early Detection in Fish Farms

Recirculating aquaculture systems (RAS) are complex systems in which there is an interaction between the fish biomass and water chemistry, where small variations in the environment can lead to major effects in the production. Ammonia is one of the key limiting factors in RAS and its early detection in small concentrations prevents fish mortality and improves the production quality. Aiming at this background, this paper presents a low-cost fiberoptic probe for the early detection of ammonia. The sensor was based on the chemical interaction between the Oxazine 170 perchlorate layer, deposited in an uncladed polymer optical fiber (POF), and the ammonia dissolved in water. In addition, a thin metallic layer (composed by gold and palladium) was deposited in the fiber end facet and acted as a reflector for the optical signals, enabling the use of the proposed sensor in reflection mode. Different configurations of the sensor were tested, where the effects of polydimethylsiloxane (PDMS) protective layer, thermal treatments, and the use on reflection or transmission modes were compared in the assessment of ammonia concentrations in the range of 100 ppb to 900 ppb. Results showed a better performance (as a function of the sensor sensitivity and linearity) of the sensor with the annealing thermal treatment and without the PDMS layer. Then, the proposed fiberoptic probe was applied on the ammonia detection in high-salinity water, where ammonia concentrations as low as 100 ppb were detected

Highly Stretchable Polymer Optical Fiber for Mechanical Sensing in Artificial Tendons: Towards Novel Sensors for Soft Robotics

The control of tendon-driven actuators is mainly affected by the tendon behavior under stress or strain. The measurement of these parameters on artificial tendons brings benefits on the control and novel approaches for soft robotics actuators. This paper presents the development of polymer optical fiber sensors fabricated through the light spinning polymerization process (LPS-POF) in artificial tendons. This fiber has exceptionally low Young’s modulus and high strain limits, suitable for sensing applications in soft structures. Two different configurations are tested, indicating the possibility of measuring strain and stress applied in the tendon with determination coefficients of 0.996 and 0.994, respectively

POF Smart Carpet: A Multiplexed Polymer Optical Fiber-Embedded Smart Carpet for Gait Analysis

This paper presents the development of a smart carpet based on polymer optical fiber (POF) for ground reaction force (GRF) and spatio-temporal gait parameter assessment. The proposed carpet has 20 intensity variation-based sensors on one fiber with two photodetectors for acquisition, each one for the response of 10 closer sensors. The used multiplexing technique is based on side-coupling between the light sources and POF lateral sections in which one light-emitting diode (LED) is activated at a time, sequentially. Three tests were performed, two for sensor characterization and one for validation of the smart carpet, where the first test consisted of the application of calibrated weights on the top of each sensor for force characterization. In the second test, the foot was positioned on predefined points distributed on the carpet, where a mean relative error of 2.9% was obtained. Results of the walking tests on the proposed POF-embedded smart carpet showed the possibility of estimating the GRF and spatio-temporal gait parameters (step and stride lengths, cadence, and stance duration). The obtained results make possible the identification of gait events (stance and swing phases) as well as the stance duration and double support periods. The proposed carpet is a low-cost and reliable tool for gait analysis in different applications

Compensation Method for Temperature Cross-Sensitivity in Transverse Force Applications with FBG Sensors in POFs

In this paper, we investigate the influence of temperature on the transverse force response of polymer optical fiber Bragg gratings (POFBGs) inscribed in cyclic transparent fluoropolymers (CYTOPs). The gratings are imprinted in the fiber using the direct-write, plane-by-plane femtosecond laser inscription method. The temperature increase leads to a decrease in the polymer Young's modulus, which causes a sensitivity variation in the POFBG sensor for transverse force applications. The proposed technique is based on the characterization of both the sensor's response offset and the material's Young's modulus variation due to the temperature increase. Transverse force tests were performed at different temperatures (30 °C, 40 °C, 50 °C, and 60 °C) and the compensated and uncompensated responses are compared in terms of root mean squared error (RMSE). The compensated results show an RMSE lower than 3% (mean value between all tested temperatures), which is 6.7 times lower compared with the uncompensated response. In addition, the proposed compensation technique presents a maximum RMSE reduction of 16 times at 60 °C