# Design of a Variable Stiffness Spring with Human-Selectable Stiffness

## TL;DR

This paper introduces a novel variable stiffness spring that users can manually adjust, enhancing wearable robotic devices' ability to assist with different tasks and user needs by providing customizable joint torque.

## Contribution

The paper presents a new human-selectable variable stiffness spring design that allows users to easily change stiffness, improving adaptability of wearable assistive devices.

## Key findings

- Users can effectively adjust spring stiffness over a large range.
- The spring assists hip joint during leg oscillations.
- Demonstrated in a leg-swing experiment.

## Abstract

Springs are commonly used in wearable robotic devices to provide assistive joint torque without the need for motors and batteries. However, different tasks (such as walking or running) and different users (such as athletes with strong legs or the elderly with weak legs) necessitate different assistive joint torques, and therefore, springs with different stiffness. Variable stiffness springs are a special class of springs which can exert more or less torque upon the same deflection, provided that the user is able to change the stiffness of the spring. In this paper, we present a novel variable stiffness spring design in which the user can select a preferred spring stiffness similar to switching gears on a bicycle. Using a leg-swing experiment, we demonstrate that the user can increment and decrement spring stiffness in a large range to effectively assist the hip joint during leg oscillations. Variable stiffness springs with human-selectable stiffness could be key components of wearable devices which augment locomotion tasks, such as walking, running, and swimming.