Fiber Bragg grating-based three-dimensional human-robot interaction force sensor with high precision and large range for lower limb rehabilitation exoskeleton.

This study presents a three-dimensional (3-D) fiber Bragg grating (FBG) force sensor with high precision and large range, designed to measure 3-D interaction force between human lower limb and lower limb rehabilitation exoskeleton during rehabilitation training. The sensor integrates four FBG-inscribed optical fibers with an elastomer. The FBGs are arranged at 120° intervals circumferentially around the elastomer to measure 3-D force. An additional fiber serves for temperature compensation. The sensitivity and resolution of the sensor are enhanced by introducing a simply supported beam structure and optimizing key size parameters of FBG sensor. This study proposes a novel measurement method for the accurate decoupling of 3-D force, which integrates temperature compensation and differential decoupling. The implementation of this combined technical approach is enabled by the specially designed sensor mechanical structure and arrangement of FBGs. The FBG sensor is designed to measure force ranging from 0 to 300 N in the Z direction and -50 to 50 N in the X and Y directions. Calibration experiments were conducted to determine the sensor's sensitivity coefficients, achieving resolutions of 0.046 N, 0.047 N, and 0.093 N in the X, Y and Z directions, respectively. The validation experiments demonstrated that the proposed measurement method achieves favorable decoupling performance. In addition, human-robot interaction force measurement experiment was conducted to verify the sensor's dynamic response capability to walking speed variation. A subsequent 600-gait-cycle test at 3.5 km/h on a sensor-integrated lower limb rehabilitation exoskeleton yielded a coefficient of variation below 10%, confirming the sensor's stability in practice, as well as the effectiveness of the temperature compensation scheme.