An Uncontrolled Toy That Can Walk But Cannot Stand Still

TL;DR

This paper introduces a simple gravity-powered two-leg toy that can walk stably without control systems, demonstrating passive dynamic stability in motion despite being statically unstable, and explores nonholonomic mechanics in balance.

Contribution

It presents the first McGeer-like passive-dynamic walker that is statically unstable but stable in motion, challenging existing design assumptions.

Findings

The toy walks downhill powered solely by gravity.

It remains stable during motion despite being statically unstable.

Simulations did not predict the observed stability.

Abstract

We built a simple two-leg toy that can walk stably with no control system. It walks downhill powered only by gravity. It seems to be the first McGeer-like passive-dynamic walker that is statically unstable in all standing positions, yet is stable in motion. It is one of few known mechanical devices that are stable near a statically unstable configuration but do not depend on spinning parts. Its design is loosely based on simulations which do not predict its observed stability. Its motion highlights the possible role of uncontrolled nonholonomic mechanics in balance.