Instantaneous Capture Input for Balancing the Variable Height Inverted Pendulum

TL;DR

This paper extends the concept of the instantaneous capture point to variable-height inverted pendulums by introducing the instantaneous capture input, enabling improved balance control and capturability analysis for legged robots.

Contribution

It introduces the instantaneous capture input (ICI) for variable-height inverted pendulums, providing a new analytical framework and an explicit ICI-based controller with online optimal feedback gains.

Findings

The ICI can be computed as a function of the system state.

The ICI-based control policy stabilizes the VHIP system.

Simulations show improved capturability maintenance over existing methods.

Abstract

Balancing is a fundamental need for legged robots due to their unstable floating-base nature. Balance control has been thoroughly studied for simple models such as the linear inverted pendulum thanks to the concept of the instantaneous capture point (ICP), yet the constant center of mass height assumption limits the application. This paper explores balancing of the variable-height inverted pendulum (VHIP) model by introducing the \emph{instantaneous capture input} (ICI), an extension of the ICP based on its key properties. Namely, the ICI can be computed as a function of the state, and when this function is used as the control policy, the ICI is rendered stationary and the system will eventually come to a stop. This characterization induces an analytical region of capturable states for the VHIP, which can be used to conceptually guide where to step. To further address state and control constraints during recovery, we present and theoretically analyze an explicit ICI-based controller with online optimal feedback gains. Simulations demonstrate the validity of our controller for capturability maintenance compared to an approach based on the divergent component of motion.