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Design of a Variable Stiffness Quasi-Direct Drive Cable-Actuated Tensegrity Robot

Jonathan Mi, Wenzhe Tong, Yilin Ma, Xiaonan Huang

TL;DR

This work tackles state estimation and payload adaptability in tensegrity robots by introducing a modular three-bar tensegrity powered by Quasi-Direct Drive (QDD) cable actuators and low-stretch Dyneema cables. The design enables accurate proprioception without external force/torque sensors and offers on-the-fly stiffness tuning, demonstrated by cable-length estimation errors under $<1\%$ of the bar length and variable stiffness control up to $7\times$ the minimum. The authors detail mechanical, electrical, and software architectures, show fabrication and experimental validation, and present a path toward autonomous operation through an open, modular exoskeleton design. The platform supports future sensing and computing modules, enabling robust, adaptable tensegrity robots for constrained and unpredictable environments.

Abstract

Tensegrity robots excel in tasks requiring extreme levels of deformability and robustness. However, there are challenges in state estimation and payload versatility due to their high number of degrees of freedom and unconventional shape. This paper introduces a modular three-bar tensegrity robot featuring a customizable payload design. Our tensegrity robot employs a novel Quasi-Direct Drive (QDD) cable actuator paired with low-stretch polymer cables to achieve accurate proprioception without the need for external force or torque sensors. The design allows for on-the-fly stiffness tuning for better environment and payload adaptability. In this paper, we present the design, fabrication, assembly, and experimental results of the robot. Experimental data demonstrates the high accuracy cable length estimation (<1% error relative to bar length) and variable stiffness control of the cable actuator up to 7 times the minimum stiffness for self support. The presented tensegrity robot serves as a platform for future advancements in autonomous operation and open-source module design.

Design of a Variable Stiffness Quasi-Direct Drive Cable-Actuated Tensegrity Robot

TL;DR

This work tackles state estimation and payload adaptability in tensegrity robots by introducing a modular three-bar tensegrity powered by Quasi-Direct Drive (QDD) cable actuators and low-stretch Dyneema cables. The design enables accurate proprioception without external force/torque sensors and offers on-the-fly stiffness tuning, demonstrated by cable-length estimation errors under of the bar length and variable stiffness control up to the minimum. The authors detail mechanical, electrical, and software architectures, show fabrication and experimental validation, and present a path toward autonomous operation through an open, modular exoskeleton design. The platform supports future sensing and computing modules, enabling robust, adaptable tensegrity robots for constrained and unpredictable environments.

Abstract

Tensegrity robots excel in tasks requiring extreme levels of deformability and robustness. However, there are challenges in state estimation and payload versatility due to their high number of degrees of freedom and unconventional shape. This paper introduces a modular three-bar tensegrity robot featuring a customizable payload design. Our tensegrity robot employs a novel Quasi-Direct Drive (QDD) cable actuator paired with low-stretch polymer cables to achieve accurate proprioception without the need for external force or torque sensors. The design allows for on-the-fly stiffness tuning for better environment and payload adaptability. In this paper, we present the design, fabrication, assembly, and experimental results of the robot. Experimental data demonstrates the high accuracy cable length estimation (<1% error relative to bar length) and variable stiffness control of the cable actuator up to 7 times the minimum stiffness for self support. The presented tensegrity robot serves as a platform for future advancements in autonomous operation and open-source module design.
Paper Structure (24 sections, 8 equations, 13 figures, 1 table)

This paper contains 24 sections, 8 equations, 13 figures, 1 table.

Table of Contents

  1. Introduction
  2. Related Works
  3. Mechanical Design
  4. Dual Drive Unit
  5. Actuator Design
  6. Cable Spool
  7. Cable Selection
  8. Endcap Unit
  9. Compliant Cap
  10. Cable Router and Cable Anchor
  11. Manufacturing
  12. Electrical Design
  13. Robot Power
  14. Motor Controller
  15. Additional Sensors
  16. ...and 9 more sections

Figures (13)

  • Figure 1: The assembled three-bar tensegrity robot. The robot tether enters through the triangular face of the structure and connects to the middle of each bar.
  • Figure 2: Internal view of a single tensegrity robot bar. Each bar contains two endcaps and two drive units. The inertial measurement unit (IMU) is mounted on the bottom of one of the drive units and has its z-axis aligned with the central axis of the bar. The placeholder unit inserted between the endcap and the drive unit demonstrates a feasible location for an additional module. Additional modules can also be inserted in the space between the two drive units.
  • Figure 3: Cutaway view of the dual drive unit model. The red and teal strings are the actuated cables. The right side of the image shows a close up of the belt driven, dead-axle, grooved cable spool design. The motor controllers are located at the base of the unit.
  • Figure 4: Simplified 2D physics model for a single bar held in equilibrium by a cable and ground contact.
  • Figure 5: A sliced open compliant cap shows the internal structure. The low density gyroid infill allows for uniform compliance in all directions. The part is printed with 95A TPU filament.
  • ...and 8 more figures