A Compliant 3-Axis Fiber-Optic Force Sensor for Biomechanical Measurement

This paper presents the development of a flexible, multi-axis, intensity modulated-based fiber-optic force sensor for concurrently measuring normal and shear forces. The proposed sensor was prototyped to measure the three force components by monitoring the variation of the light intensity induced by a deformation as a result of the applied force. One end of the sensor incorporates three orthogonal reflective planes. The other end brings three pairs of fibers; one fiber connected to an LED, and the other to a light-to-voltage (LTV) converter in each pair. Upon the application of the force, the distance between the planes and the fiber tips changes, thus, the LTV voltage changes, enabling the simultaneous measurement of forces along three normal axes utilizing only one set of force measurement unit. The fabricated sensor was tested in both static and dynamic loading conditions as the experimental results have confirmed that the prototype has the capability to accurately measure the normal and shear forces in real time ranging from 0 to 1000 N and 0 to 140 N along the z, x, and y axes, respectively. The feasible applications of the sensor are ground reaction force measurements and robot–human collision detection. Sensor performance was evaluated for the cross-talk effects, which were found to be less than 5%. A nonlinear Hammerstein–Winer model is proposed to characterize the linear and nonlinear behavior of the sensor. The optimized results have shown a reduction of over 40% in the root mean-square errors in comparison with the linear estimation models.