Compact Optical Single-axis Joint Torque Sensor Using Redundant Photo-Reflectors and Quadratic-Programming Calibration

2026-03-16

AI Summary

This study addresses the insufficient accuracy of joint torque estimation in collaborative robots within low-torque regimes, primarily caused by gearbox static friction and nonlinear current-to-torque relationships. To overcome this limitation, the authors propose a non-contact optical joint torque sensor based on micro-deformation detection of an elastic structure. The design incorporates a quadrature-redundant array of optical reflective sensors to enhance sensitivity and signal-to-noise ratio, complemented by quadratic programming-based calibration, temperature drift compensation, and rational function fitting. Experimental results demonstrate exceptional performance, with a full-scale maximum error of only 0.083%, an RMS error of 0.0266 Nm, and a 3σ resolution of 0.0224 Nm at 1 kHz—representing a 2.14-fold improvement over conventional least-squares methods. The system exhibits superior precision and robustness to disturbances in low-torque tracking and admittance control tasks.

Technology Category

• Intelligent Robots: State Estimation
• Computer Vision: Low Level & Physics-based Vision
• Search and Optimization: Mixed Discrete/Continuous Search

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Experiences and lessons learnt from Web-based algorithms and system deployments
• Graph Algorithms and Modeling for the Web: Efficient manipulation of static and dynamic Web-related graphs
• Responsible Web: Human-perceived consequences of algorithmic deployment on the web

Abstract

This study proposes a non-contact photo-reflector-based joint torque sensor for precise joint-level torque control and safe physical interaction. Current-sensor-based torque estimation in many collaborative robots suffers from poor low-torque accuracy due to gearbox stiction/friction and current-torque nonlinearity, especially near static conditions. The proposed sensor optically measures micro-deformation of an elastic structure and employs a redundant array of photo-reflectors arranged in four directions to improve sensitivity and signal-to-noise ratio. We further present a quadratic-programming-based calibration method that exploits redundancy to suppress noise and enhance resolution compared to least-squares calibration. The sensor is implemented in a compact form factor (96 mm diameter, 12 mm thickness). Experiments demonstrate a maximum error of 0.083%FS and an RMS error of 0.0266 Nm for z-axis torque measurement. Calibration tests show that the proposed calibration achieves a 3 sigma resolution of 0.0224 Nm at 1 kHz without filtering, corresponding to a 2.14 times improvement over the least-squares baseline. Temperature chamber characterization and rational fitting based compensation mitigate zero drift induced by MCU self heating and motor heat. Motor-level validation via torque control and admittance control confirms improved low torque tracking and disturbance robustness relative to current-sensor-based control.

Problem

• joint torque sensing
• low-torque accuracy
• gearbox friction
• current-torque nonlinearity
• static conditions

Innovation

• optical torque sensor
• redundant photo-reflectors
• quadratic-programming calibration
• micro-deformation measurement
• low-torque accuracy