Design, Development and Calibration of a Lightweight, Compliant Six-Axis Optical Force/Torque Sensor

This paper introduces the fabrication of a six degree-of-freedom force and torque sensor based on fiber-optic sensing technology and its novel calibration methodology. The sensor is cost effective, lightweight, and flexible with a large force and torque measurement range suitable for biomechanics and rehabilitation systems particularly when a wearable sensing system is desired. Six fiber-optic sensing elements are used to detect three main forces Fx, Fy, and Fz, and three main torques Tx, Ty, and Tz. Sensor data were collected by applying dynamic forces and torques with various magnitudes, directions, and frequencies and compared with measurements obtained from a standard force and torque reference. The proposed calibration procedure is intended to reduce errors stemmed from a nonlinear force-deformation relationship and to increase the estimation speed by splitting the calibration into two estimation models: a linear model, based on a standard least squares method (LSM) to estimate the linear portion, and a nonlinear decision trees' model (DT) to estimate the residuals. Both the models work simultaneously as a single calibration system named least squares decision trees LSDT. Using LSDT, the estimation speed increased by 55.17%, and the root mean square errors (RMSEs) reduced to 0.53%. In comparison, each model separately had a RMSEs of 1.26% and 4.70% for the DT and the LSM, respectively.