Analysis, Design, and Preliminary Evaluation of a Parallel Elastic Actuator for Power-Efficient Walking Assistance

Yinbo Li,Ze Yang,Steffen Leonhardt,Zihan Tang,Xinyu Guan,Linhong Ji,Zhibin Li,Bernhard Penzlin

doi:10.1109/access.2020.2993183

Abstract

This paper introduces the analysis, design and preliminary evaluation of a self-integrated parallel elastic actuator (PEA) with an electric motor and a flat spiral spring in parallel to drive the hip joint of lower limb exoskeletons for power-efficient walking assistance. Firstly, we quantitatively analyze the reason why the parallel elasticity (PE) placed at the sagittal hip joint can reduce the motor power requirement during walking assistance, which contributes to the theory of PEA development and application. The design of the PEA is then introduced in detail. The novelty of the design is that the actuator is reduced in size by integrating the spring into the motor, and the requirement of spring stiffness is significantly reduced by placing the spring directly parallel to the motor shaft. Furthermore, both the simulation based on dynamic modeling and benchtop experiment are conducted preliminarily to evaluate the performance of the PEA with nine spring stiffnesses in a range from 0 – 5.29 mN $\cdot \text{m}$ /rad regarding two indexes including the average and maximum positive electrical power of the motor. Their results show that the two indexes become smaller when the PE is attached and decrease as the spring stiffness increases. When the PE with a stiffness of 5.29 mN $\cdot \text{m}$ /rad is attached, the actuator obtains the largest reduction rate of 11.99 and 16.84 % in the average (root mean square) and maximum positive electrical power of the motor in the simulation and 10.3 and 26.25 % in the experiment, respectively. Those results provide evidence for the applicability of the newly designed PEA in driving a lower limb exoskeleton with high power efficiency during walking assistance for paraplegic patients with complete loss in walking ability.

Highlights

Traditional rigid actuators (RAs), as the key units of walking assistance devices such as lower limb exoskeletons (LLEs), consume a lot of power, leading to a high power requirement
The results of the simulation and experiment demonstrate that both indexes tended to decrease substantially with the increase of k and were smaller when the parallel elasticity (PE) were attached than when they were not
Both indexes reached the minimum at k = 5.29 mN·m/rad in the simulation and experiment

Summary

Introduction

Traditional rigid actuators (RAs) (e.g. electric motors), as the key units of walking assistance devices such as lower limb exoskeletons (LLEs), consume a lot of power, leading to a high power requirement. Since the gait is a periodic process, compliant elements can store the energy wasted as negative work in the period when the motor must brake the load actively and recycle it as positive work for accelerations in the opposite direction Such characteristics of compliant elements can be used to enrich the dynamics of traditional electric motors [2], [3] for walking assistance [4]. Compliant actuation, such as elastic actuators (EAs), adding the elasticity element to the RA, is widely studied in robotic applications. The EAs are generally divided into series (SEAs) and parallel EAs (PEAs), regarding the

Results

Discussion

Conclusion