Parameter Optimization of Optical Six-Axis Force/Torque Sensor for Legged Robots

TL;DR

This work presents a compact, non-contact optical six-axis force/torque sensor based on photo-couplers for legged robots, addressing durability and cost limitations of traditional strain-gauge sensors. A T-beam elastomer mechanism and a minimal PCB are used to convert forces and moments into optical displacements, with a FEM-driven global optimization that balances isotropy and sensitivity within a 40 mm envelope. The chosen objective combines matrix conditioning and norm-based sensitivity, yielding an optimized design later validated through fabrications and extensive quadruped experiments against a reference sensor. Results show substantially improved durability and resolution (notably in $F_z$) with a low prototype cost (under $300) and high sampling rate ($\sim$5 kHz), supporting broad applicability in dynamic robotic environments.

Abstract

This paper introduces a novel six-axis force/torque sensor tailored for compact and lightweight legged robots. Unlike traditional strain gauge-based sensors, the proposed non-contact design employs photocouplers, enhancing resistance to physical impacts and reducing damage risk. This approach simplifies manufacturing, lowers costs, and meets the demands of legged robots by combining small size, light weight, and a wide force measurement range. A methodology for optimizing sensor parameters is also presented, focusing on maximizing sensitivity and minimizing error. Precise modeling and analysis of objective functions enabled the derivation of optimal design parameters. The sensor's performance was validated through extensive testing and integration into quadruped robots, demonstrating alignment with theoretical modeling. The sensor's precise measurement capabilities make it suitable for diverse robotic environments, particularly in analyzing interactions between robot feet and the ground. This innovation addresses existing sensor limitations while contributing to advancements in robotics and sensor technology, paving the way for future applications in robotic systems.

Figures (12)

• Figure 1: a) Configuration and Principle of Proposed Sensor. 3 Vertical Photo-Couplers for $F_z, M_x, M_y$ and 3 Horizontal Photo-Couplers for $F_x, F_y, M_z$ (b) Parameters of Proposed Sensor.: $r$: radius of loading table, $l_1, l_2, b_1, b_2, h$: Dimension of T-beam, and $r_{s1}$ and $r_{s2}$: Radius of Vertical Photocouplers and Horizontal Photocouplers
• Figure 2: Elastomer deformation according to the force and moment applied along each axis: (a) Deformation when applying z-axis force ($F_z$) (b) Deformation when applying y-axis moment ($M_y$) (c) Deformation when applying x-axis force ($F_x$) (d) Deformation when applying z-axis moment ($M_z$)
• Figure 3: When deformation occurs along the negative y-axis and the horizontal sensor is positioned at $r_{s2}$, the deformation measured by the horizontal sensor is $\Delta d_5$.
• Figure 4: Sensor Configuration by Objective Function using FEM
• Figure 5: Sensor Configuration by Objective Function using FEM: (a) Each Part of Sensors and Assembly (b) Calibration Method(Reference Sensor: ATI MINI85)