Natural Gradient Gaussian Approximation Filter on Lie Groups for Robot State Estimation

TL;DR

This paper introduces NANO-L, a novel Lie group-based filter for robot state estimation that avoids linearization, uses natural gradient optimization, and achieves lower error in experiments.

Contribution

It reformulates manifold filtering as a Gaussian parameter optimization problem and develops a natural gradient scheme for improved accuracy and efficiency.

Findings

NANO-L achieves approximately 40% lower estimation error in hardware tests.

The covariance update admits an exact closed-form solution for invariant observation models.

NANO-L maintains computational efficiency comparable to existing filters.

Abstract

Accurate state estimation for robotic systems evolving on Lie group manifolds, such as legged robots, is a prerequisite for achieving agile control. However, this task is challenged by nonlinear observation models defined on curved manifolds, where existing filters rely on local linearization in the tangent space to handle such nonlinearity, leading to accumulated estimation errors. To address this limitation, we reformulate manifold filtering as a parameter optimization problem over a Gaussian-distributed increment variable, thereby avoiding linearization. Under this formulation, the increment can be mapped to the Lie group through the exponential operator, where it acts multiplicatively on the prior estimate to yield the posterior state. We further propose a natural gradient optimization scheme for solving this problem, whose iteration process leverages the Fisher information matrix of the increment variable to account for the curvature of the tangent space. This results in an iterative algorithm named the Natural Gradient Gaussian Approximation on Lie Groups (NANO-L) filter. Leveraging the perturbation model in Lie derivative, we prove that for the invariant observation model widely adopted in robotic localization tasks, the covariance update in NANO-L admits an exact closed-form solution, eliminating the need for iterative updates thus improving computational efficiency. Hardware experiments on a Unitree GO2 legged robot operating across different terrains demonstrate that NANO-L achieves approximately 40% lower estimation error than commonly used filters at a comparable computational cost.