Contact Sensing via Joint Torque Sensors and a Force/Torque Sensor for Legged Robots

TL;DR

This work tackles contact detection and localization along robot legs by combining distributed joint torque sensing with a single hip-mounted force–torque sensor within a generalized momentum-based observer. It derives a momentum-based framework where $\dot{\mathbf{P}} = \mathbf{u} - \boldsymbol{\tau}_{ext}$ and uses residuals to detect collisions and identify the contacted link, leveraging a base wrench to localize the contact. Hardware validation on a 2-DoF leg shows sub-centimeter localization accuracy and force errors below $0.2\,\text{N}$, while the joint torque sensors achieve $96.4\%$ accuracy compared to ground truth and outperform motor-current estimates. The approach enables safe, high-bandwidth collision sensing along the entire leg, with potential for improved robustness in cluttered environments without heavy friction modeling.

Abstract

This paper presents a method for detecting and localizing contact along robot legs using distributed joint torque sensors and a single hip-mounted force-torque (FT) sensor using a generalized momentum-based observer framework. We designed a low-cost strain-gauge-based joint torque sensor that can be installed on every joint to provide direct torque measurements, eliminating the need for complex friction models and providing more accurate torque readings than estimation based on motor current. Simulation studies on a floating-based 2-DoF robot leg verified that the proposed framework accurately recovers contact force and location along the thigh and shin links. Through a calibration procedure, our torque sensor achieved an average 96.4% accuracy relative to ground truth measurements. Building upon the torque sensor, we performed hardware experiments on a 2-DoF manipulator, which showed sub-centimeter contact localization accuracy and force errors below 0.2 N.

Figures (7)

• Figure 1: Custom low-cost strain-gauge joint torque sensor. (Left) Fabricated sensor with strain gauges. (Right) Finite-Element Analysis (FEA) of the custom joint torque sensor under 0.4 Nm load.
• Figure 2: System schematic depicting data flow for contact detection: (a) motor hardware with joint torque sensors, data acquisition system, and encoders, which feed into (b) a generalized momentum observer sensing algorithm, subsequently input with a 6-DoF force-torque (FT) sensor data for contact detection, outputting estimated contact force and location.
• Figure 3: Sensor configuration rationale for multi-link collision detection. A planar point contact has 3 degrees of freedom (Fx, Fz, and contact position $p$), but a 2-DoF serial chain provides only 2 joint torque measurements ($\tau_1$, $\tau_2$), creating an underdetermined system. The base-mounted FT sensor provides the necessary additional measurements for complete contact localization while remaining effective for contacts on any link.
• Figure 4: Results of Fixed-Base Simulation Parametric Sweep of Configurations.
• Figure 5: Collision detection for a floating-base robot. (a) Illustration of the setup with red external force and green ground reaction force. (b) and (c) Show actual vs. estimated contact force magnitude and location, yellow regions indicate post-collision response (t $\geq$ 0.5s).