# Compliance Shaping for Control of Strength Amplification Exoskeletons with Elastic Cuffs

## TL;DR

This paper presents a novel compliance shaping approach for strength amplification exoskeletons with elastic cuffs, enhancing robustness and stability through a double compliance design and disturbance observer, validated on a single-degree-of-freedom system.

## Contribution

It introduces the double compliance shaping method combining cuff and SEA compliance, along with a disturbance observer, to improve stability and robustness in strength amplification exoskeletons.

## Key findings

- The method enhances robustness to human behaviors.
- The controller effectively amplifies operator strength.
- Validation on a single DOF exoskeleton demonstrates feasibility.

## Abstract

Exoskeletons which amplify the strength of their operators can enable heavy-duty manipulation of unknown objects. However, this type of behavior is difficult to accomplish; it requires the exoskeleton to sense and amplify the operator's interaction forces while remaining stable. But, the goals of amplification and robust stability when connected to the operator fundamentally conflict. As a solution, we introduce a design with a spring in series with the force sensitive cuff. This allows us to design an exoskeleton compliance behavior which is nominally passive, even with high amplification ratios. In practice, time delay and discrete time filters prevent our strategy from actually achieving passivity, but the designed compliance still makes the exoskeleton more robust to spring-like human behaviors. Our exoskeleton is actuated by a series elastic actuator (SEA), which introduces another spring into the system. We show that shaping the cuff compliance for the exoskeleton can be made into approximately the same problem as shaping the spring compliance of an SEA. We therefore introduce a feedback controller and gain tuning method which takes advantage of an existing compliance shaping technique for SEAs. We call our strategy the "double compliance shaping" method. With large amplification ratios, this controller tends to amplify nonlinear transmission friction effects, so we additionally propose a "transmission disturbance observer" to mitigate this drawback. Our methods are validated on a single-degree-of-freedom elbow exoskeleton.