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Postural Virtual Fixtures for Ergonomic Physical Interactions with Supernumerary Robotic Bodies

Theodora Kastritsi, Marta Lagomarsino, Arash Ajoudani

TL;DR

The paper tackles ergonomic risk in SRB-assisted pHRI by introducing Ergonomics Postural Virtual Fixtures (PVFs) that provide kinesthetic feedback when a non-ergonomic posture is detected. The approach combines an admittance-based end-effector model with a god-object representation, online continuous RULA-based ergonomics assessment, and dynamic base adaptation on a floating SRB to improve coordination and safety. Key contributions include formal passivity guarantees, a continuous ergonomics score $a\in[0,1]$, and null-space coordination that prevents base interference while preserving task flexibility; validated through two multi-subject experiments with 14 participants showing improved posture and learning effects. The framework offers substantial practical impact for safer, ergonomically aware human–robot collaboration using mobile SRBs, with plans to extend to lower-body metrics, sensor fusion, and autonomous tracking.

Abstract

Conjoined collaborative robots, functioning as supernumerary robotic bodies (SRBs), can enhance human load tolerance abilities. However, in tasks involving physical interaction with humans, users may still adopt awkward, non-ergonomic postures, which can lead to discomfort or injury over time. In this paper, we propose a novel control framework that provides kinesthetic feedback to SRB users when a non-ergonomic posture is detected, offering resistance to discourage such behaviors. This approach aims to foster long-term learning of ergonomic habits and promote proper posture during physical interactions. To achieve this, a virtual fixture method is developed, integrated with a continuous, online ergonomic posture assessment framework. Additionally, to improve coordination between the operator and the SRB, which consists of a robotic arm mounted on a floating base, the position of the floating base is adjusted as needed. Experimental results demonstrate the functionality and efficacy of the ergonomics-driven control framework, including two user studies involving practical loco-manipulation tasks with 14 subjects, comparing the proposed framework with a baseline control framework that does not account for human ergonomics.

Postural Virtual Fixtures for Ergonomic Physical Interactions with Supernumerary Robotic Bodies

TL;DR

The paper tackles ergonomic risk in SRB-assisted pHRI by introducing Ergonomics Postural Virtual Fixtures (PVFs) that provide kinesthetic feedback when a non-ergonomic posture is detected. The approach combines an admittance-based end-effector model with a god-object representation, online continuous RULA-based ergonomics assessment, and dynamic base adaptation on a floating SRB to improve coordination and safety. Key contributions include formal passivity guarantees, a continuous ergonomics score , and null-space coordination that prevents base interference while preserving task flexibility; validated through two multi-subject experiments with 14 participants showing improved posture and learning effects. The framework offers substantial practical impact for safer, ergonomically aware human–robot collaboration using mobile SRBs, with plans to extend to lower-body metrics, sensor fusion, and autonomous tracking.

Abstract

Conjoined collaborative robots, functioning as supernumerary robotic bodies (SRBs), can enhance human load tolerance abilities. However, in tasks involving physical interaction with humans, users may still adopt awkward, non-ergonomic postures, which can lead to discomfort or injury over time. In this paper, we propose a novel control framework that provides kinesthetic feedback to SRB users when a non-ergonomic posture is detected, offering resistance to discourage such behaviors. This approach aims to foster long-term learning of ergonomic habits and promote proper posture during physical interactions. To achieve this, a virtual fixture method is developed, integrated with a continuous, online ergonomic posture assessment framework. Additionally, to improve coordination between the operator and the SRB, which consists of a robotic arm mounted on a floating base, the position of the floating base is adjusted as needed. Experimental results demonstrate the functionality and efficacy of the ergonomics-driven control framework, including two user studies involving practical loco-manipulation tasks with 14 subjects, comparing the proposed framework with a baseline control framework that does not account for human ergonomics.
Paper Structure (33 sections, 1 theorem, 38 equations, 17 figures, 2 tables)

This paper contains 33 sections, 1 theorem, 38 equations, 17 figures, 2 tables.

Table of Contents

  1. INTRODUCTION
  2. Related Works
  3. Postural Ergonomics:
  4. Virtual Fixture:
  5. Mobile Manipulators
  6. Contributions
  7. PROPOSED METHODOLOGY
  8. Postural Virtual Fixtures
  9. Variable Damping
  10. Power-Based Variable Damping:
  11. Ergonomics-Based Variable Damping:
  12. Online Continuous Ergonomics Assessment
  13. Supernumerary Robotic Body Control with Base Adaptation
  14. EXPERIMENTS
  15. Experimental Setup
  16. ...and 18 more sections

Key Result

Theorem 1

For the system eq:adm_x2y, eq:dyn_god, utilizing the control term eq:u_c, the following statements are valid under the exertion of the human force $\mathbf F_h$:

Figures (17)

  • Figure 1: Block diagram illustrating the proposed methodology, outlining the key components and their interactions.
  • Figure 2: Visualization of the sigmoid function $h$ used for for smooth switching.
  • Figure 3: Analysis of human joint angles: (a) shoulder abduction/adduction, (b) shoulder flexion/extension, (c) shoulder internal/external rotation, (d) elbow flexion/extension, and (e) sagittal bending, across five movement scenarios. The figure illustrates key points identified by perception sensors, representing joint positions during each movement.
  • Figure 4: Virtual representation of the ergonomics sub-factors.
  • Figure 5: Visualization of the repulsive vector when the user is in close proximity to the robotic platform.
  • ...and 12 more figures

Theorems & Definitions (6)

  • Remark 1
  • Theorem 1
  • Remark 2
  • Remark 3
  • Remark 4
  • Remark 5