# Complex Stiffness Model of Physical Human-Robot Interaction: Implications for Control of Performance Augmentation Exoskeletons

## TL;DR

This paper introduces a complex stiffness model for human joint dynamics in exoskeletons, incorporating hysteretic damping, validated through experiments, and demonstrates how this model can enhance control strategies for performance augmentation.

## Contribution

The paper proposes a novel complex stiffness model with hysteretic damping for human joints, validated experimentally, and shows how it can improve exoskeleton control performance.

## Key findings

- Hysteretic damping improves modeling accuracy over classical viscous damping.
- A linear relationship exists between hysteretic damping and real stiffness.
- A fractional order controller exploits hysteretic damping for better performance.

## Abstract

Human joint dynamic stiffness plays an important role in the stability of performance augmentation exoskeletons. In this paper, we consider a new frequency domain model of the human joint dynamics which features a complex value stiffness. This complex stiffness consists of a real stiffness and a hysteretic damping. We use it to explain the dynamic behaviors of the human connected to the exoskeleton, in particular the observed non-zero low frequency phase shift and the near constant damping ratio of the resonant as stiffness and inertia vary. We validate this concept by experimenting with an elbow-joint exoskeleton testbed on a subject while modifying joint stiffness behavior, exoskeleton inertia, and strength augmentation gains. We compare three different models of elbow-joint dynamic stiffness: a model with real stiffness, viscous damping and inertia, a model with complex stiffness and inertia, and a model combining the previous two models. Our results show that the hysteretic damping term improves modeling accuracy, using a statistical F-test. Moreover this improvement is statistically more significant than using classical viscous damping term. In addition, we experimentally observe a linear relationship between the hysteretic damping and the real part of the stiffness which allows us to simplify the complex stiffness model as a 1-parameter system. Ultimately, we design a fractional order controller to demonstrate how human hysteretic damping behavior can be exploited to improve strength amplification performance while maintaining stability.