A Miniature High-Resolution Tension Sensor Based on a Photo-Reflector for Robotic Hands and Grippers

TL;DR

This paper introduces a compact, high-resolution tension sensor based on a photo-reflector, suitable for robotic hands, offering improved sensitivity, resolution, and ease of integration over previous designs.

Contribution

A novel miniature tension sensor utilizing a photo-reflector with enhanced resolution, mechanical durability, and simplified assembly for robotic applications.

Findings

Achieves 9.9 mN resolution, over 14-bit accuracy.

Demonstrates force measurement RMSE of 0.455 N.

Enables precise force control with RMSE as low as 0.073 N.

Abstract

This paper presents a miniature tension sensor using a photo-reflector, designed for compact tendon-driven grippers and robotic hands. The proposed sensor has a small form factor of 13~mm x 7~mm x 6.5~mm and is capable of measuring tensile forces up to 200~N. A symmetric elastomer structure incorporating fillets and flexure hinges is designed based on Timoshenko beam theory and verified via FEM analysis, enabling improved sensitivity and mechanical durability while minimizing torsional deformation. The sensor utilizes a compact photo-reflector (VCNT2020) to measure displacement in the near-field region, eliminating the need for light-absorbing materials or geometric modifications required in photo-interrupter-based designs. A 16-bit analog-to-digital converter (ADC) and CAN-FD (Flexible Data-rate) communication enable efficient signal acquisition with up to 5~kHz sampling rate. Calibration experiments demonstrate a resolution of 9.9~mN (corresponding to over 14-bit accuracy) and a root mean square error (RMSE) of 0.455~N. Force control experiments using a twisted string actuator and PI control yield RMSEs as low as 0.073~N. Compared to previous research using photo-interrupter, the proposed method achieves more than tenfold improvement in resolution while also reducing nonlinearity and hysteresis. The design is mechanically simple, lightweight, easy to assemble, and suitable for integration into robotic and prosthetic systems requiring high-resolution force feedback.