# Cardioid oscillator-based pattern generator for imitating the time-ratio-asymmetrical behavior of the lower limb exoskeleton

## TL;DR

This paper introduces a pattern generator using cardioid oscillators to mimic the asymmetric timing of human lower limb movement for exoskeleton control.

## Contribution

A novel pattern generator with four degrees of freedom based on self-excited cardioid oscillators for imitating asymmetric gait characteristics.

## Key findings

- The proposed pattern generator exhibits self-excitation and asymmetric time ratios similar to natural human gait.
- Simulated trajectories match experimental joint trajectories, showing phase locking and self-adjustment capabilities.
- The generator can serve as a reference model for lower limb exoskeleton control algorithms.

## Abstract

Periodicity, self-excitation, and time ratio asymmetry are the fundamental characteristics of the human gait. In order to imitate these mentioned characteristics, a pattern generator with four degrees of freedom is proposed based on cardioid oscillators developed by the authors.

The proposed pattern generator is composed of four coupled cardioid oscillators, which are self-excited and have asymmetric time ratios. These oscillators are connected with other oscillators through coupled factors. The dynamic behaviors of the proposed oscillators, such as phase locking, time ratio, and self-excitation, are analyzed via simulations by employing the harmonic balance method. Moreover, for comparison, the simulated trajectories are compared with the natural joint trajectories measured in experiments.

Simulation and experimental results show that the behaviors of the proposed pattern generator are similar to those of the natural lower limb. It means the simulated trajectories from the generator are self-excited without any additional inputs and have asymmetric time ratios. Their phases are locked with others. Moreover, the proposed pattern generator can be applied as the reference model for the lower limb exoskeleton controlling algorithm to produce self-adjusted reference trajectories.