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Zippy: The smallest power-autonomous bipedal robot

Steven Man, Soma Narita, Josef Macera, Naomi Oke, Aaron M. Johnson, Sarah Bergbreiter

TL;DR

The paper addresses centimeter-scale bipedal locomotion with limited actuation by leveraging quasi-passive walking principles. It introduces Zippy, a 3.6 cm tall, self-contained biped with a single hip actuator, curved feet, and open-loop control, achieving a forward speed of $25\ \text{cm/s}$ (10 leg lengths per second) and enabling turning, skipping gaits, and step/rough-terrain traversal. Key contributions include validating ellipsoidal feet for stability, demonstrating a fast, self-contained small-scale walker, and quantifying energy efficiency with a minimum cost of transport around $11.2$; it also compares design-rule modifications against a scaled baseline (Scaled Mugatu). The results highlight the practicality of scaling passive-dynamic concepts to ultra-small robots, offering potential for inspection or search-and-rescue in confined spaces, while noting the need for sensor feedback (e.g., IMU, vision) to improve robustness and enable autonomous coordination. The work thus advances ultra-compact power-autonomous bipeds and provides a foundation for future closed-loop control and multi-robot deployments in constrained environments.

Abstract

Miniaturizing legged robot platforms is challenging due to hardware limitations that constrain the number, power density, and precision of actuators at that size. By leveraging design principles of quasi-passive walking robots at any scale, stable locomotion and steering can be achieved with simple mechanisms and open-loop control. Here, we present the design and control of "Zippy", the smallest self-contained bipedal walking robot at only 3.6 cm tall. Zippy has rounded feet, a single motor without feedback control, and is capable of turning, skipping, and ascending steps. At its fastest pace, the robot achieves a forward walking speed of 25 cm/s, which is 10 leg lengths per second, the fastest biped robot of any size by that metric. This work explores the design and performance of the robot and compares it to similar dynamic walking robots at larger scales.

Zippy: The smallest power-autonomous bipedal robot

TL;DR

The paper addresses centimeter-scale bipedal locomotion with limited actuation by leveraging quasi-passive walking principles. It introduces Zippy, a 3.6 cm tall, self-contained biped with a single hip actuator, curved feet, and open-loop control, achieving a forward speed of (10 leg lengths per second) and enabling turning, skipping gaits, and step/rough-terrain traversal. Key contributions include validating ellipsoidal feet for stability, demonstrating a fast, self-contained small-scale walker, and quantifying energy efficiency with a minimum cost of transport around ; it also compares design-rule modifications against a scaled baseline (Scaled Mugatu). The results highlight the practicality of scaling passive-dynamic concepts to ultra-small robots, offering potential for inspection or search-and-rescue in confined spaces, while noting the need for sensor feedback (e.g., IMU, vision) to improve robustness and enable autonomous coordination. The work thus advances ultra-compact power-autonomous bipeds and provides a foundation for future closed-loop control and multi-robot deployments in constrained environments.

Abstract

Miniaturizing legged robot platforms is challenging due to hardware limitations that constrain the number, power density, and precision of actuators at that size. By leveraging design principles of quasi-passive walking robots at any scale, stable locomotion and steering can be achieved with simple mechanisms and open-loop control. Here, we present the design and control of "Zippy", the smallest self-contained bipedal walking robot at only 3.6 cm tall. Zippy has rounded feet, a single motor without feedback control, and is capable of turning, skipping, and ascending steps. At its fastest pace, the robot achieves a forward walking speed of 25 cm/s, which is 10 leg lengths per second, the fastest biped robot of any size by that metric. This work explores the design and performance of the robot and compares it to similar dynamic walking robots at larger scales.
Paper Structure (15 sections, 2 equations, 9 figures, 3 tables)

This paper contains 15 sections, 2 equations, 9 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Methods
  3. Robot hardware design
  4. Robot control
  5. Experimental setup
  6. Effect of modifying existing design rules
  7. Locomotion performance
  8. Energetic efficiency
  9. Results
  10. Effect of modifying existing design rules
  11. Forward locomotion
  12. Turning
  13. Efficiency
  14. Step disturbance and rough terrain
  15. Conclusions

Figures (9)

  • Figure 1: The smallest self-contained biped robot (right) next to a standard 9V battery (left). The robot has one actuator at the hip and rounded feet that provide stable passive dynamics.
  • Figure 2: An annotated rendering of the Zippy robot design.
  • Figure 3: Robot frontal and sagittal plane geometry.
  • Figure 4: The robot's walking speed as a function of motor voltage amplitude and frequency. "X" corresponds to trials where the robot turns randomly despite applying the same actuation signals. 0cm/s walking speed corresponds to trials where the robot cannot make forward progress.
  • Figure 5: Sagittal plane (pitch) and frontal plane (roll) oscillation amplitudes of: (A) Scaled Mugatu walking at 11cm/s, (B) Zippy walking at 11cm/s and (C) Zippy walking at 25cm/s.
  • ...and 4 more figures