A miniature triaxial force sensor based on fiber Bragg gratings for flexible endoscopic robot

Contact force sensing is an important means to ensure the safe operation of surgical robots. This paper presents a miniature triaxial force sensor based on fiber Bragg grating (FBG) for detecting the interaction forces between the tip of flexible endoscopic surgical robots and tissue. The proposed sensor mainly consists of a force-sensitive flexure structure, a support body, and four FBG sensors. A composite hollow structure with a parallel helical slot and elliptical hinge has been utilized to design the force-sensitive flexure with an excellent linear measurement range. The four optical fibers arranged along the circumference of the flexure are configured as a tightly stretched status with their two points fixed. This configuration can improve sensitivity and avoid the drawbacks of FBG chirping and low repeatability. Finite element method-based simulation has been implemented to optimize the flexure's structural parameters for balancing the force sensor's axial and radial resolution. The prototyped sensor has been calibrated and achieves a high resolution of 0.77 mN and 0.79 mN for both lateral forces within −1 to 1 N and 1.25 mN for axial force within 0 to 2 N. Loading experiments have been conducted to validate the effectiveness of the proposed designs. The results show that the sensor can provide a high-accuracy measurement of three force components with an average RMS error of less than 3% in the total operating ranges. Such a result validates the feasibility of the designed sensor to measure the robot-tissue interaction forces.