Variable Stiffness Actuators for Wearable Applications in Gait Rehabilitation

DANIEL GOMEZ-VARGAS; DIEGO CASAS-BOCANEGRA; MARCELA MÚNERA; FLAVIO ROBERTI; RICARDO CARELLI; CARLOS CIFUENTES

Interfacing Humans and Robots for Gait Assistance and Rehabilitation

Springer Nature Switzerland AG

Año: 2021; p. 193 - 212

Currently, there are mainly two types of robots for rehabilitation and assistance: (1)platform-based robots, intended solely for the improvement of joints function, and(2) wearable devices, which can contribute in the rehabilitation of joints in stationaryscenarios, as platform-based robots, but can also improve joints performance duringgait in daily activities outside controlled environments. Therefore, this second typeof robot exhibits considerable advantages concerning stationary platforms in aspectssuch as multimodality and applicability [1, 2].In general terms, devices applied to gait rehabilitation integrate principlesimplemented in passive orthotic structures, although incorporating the robotics?benefits (i.e., energy supply using actuators, user monitoring through sensors,programmed functionality profiles, among others) [3]. In this sense, those devicesmainly aim at improving the patients? gait pattern or decrease the metabolic effortduring walking [4]. However, considering the complexity of developing roboticdevices aimed at physical interaction scenarios, wearable devices challenge inaspects such as portability, adaptability to the human body, and compliance [2, 5].On the other hand, from the capacity of assisting the human body?s movements,robotic devices require providing high torque levels during assistive scenarios[6]. Therefore, different actuation systems have been applied in those systems,intending to improve human?robot interaction and assist pathological motor functions.Specifically, current developments integrate principles based on (1) StiffActuators, (2) Serial Elastic Actuators (SEAs), and (3) Pneumatic Actuators [4].Some devices also include other mechanisms such as (4) Hydraulic Actuatorsand (5) Magnetorheological Actuators [7, 8]. Furthermore, other actuation systemswidely implemented nowadays, particularly in wearable systems, applies conceptsof (6) Variable Stiffness Actuators (VSAs) and (7) Cable-Driven Actuators [9].Devices based on pneumatic actuators have potential in aspects such as complianceand physical interaction with the user. However, this actuation type exhibitsdisadvantages related to the overweight power supply required to assist humanmovements, as well as hydraulic actuators [9, 10]. On the other hand, wearablerobots based on magnetorheological actuators include drawbacks associated with(1) the complex and heavyweight equipment implemented and (2) the high energyconsumption to achieve this principle [8].From the other actuation systems? drawbacks presented above, electrical powersupplies could be appropriate for portable devices applied in rehabilitation andassistance scenarios because of their lightweight and autonomy [11]. However, toensure this portability, those machines need to have reduced sizes and low weights,resulting in a limited torque capacity provided by the system [9]. Consequently,actuators generally include gear mechanisms to enhance the torque capabilities andassist the human body?s movements, although reducing the actuator speed response[11]. Nonetheless, the gears? inclusion also leads to non-backdrivable mechanisms,which affect the human?robot interaction [12].Within themechanical principles that use electrical power supplies, stiff actuatorsappear to be an efficient solution to assistive devices. Specifically, this actuationsystem exhibits relevant characteristics such as high provided torque and widebandwidth, which are beneficial in assistance applications [13]. Notwithstanding,stiff actuators remain the non-passive backdrivability due to the gears system,resulting in a hard physical interaction [11?13]. Likewise, for human limbs thatinvolve movements in multiple planes, designs with stiff actuators generally restrictseveral motions, inducing abnormal compensatory movements. Moreover, in termsof interaction, these actuators can present damages derivated from external forces(e.g., impacts or unexpected motions) during real applications [13].In this context, wearable devices based on cable-driven mechanisms, serieselastic actuators, and VSA are emerging to overcome the stiff systems? limitationsand preserve the actuators in interaction scenarios. These mechanical principlesinclude elements or mechanisms in the actuator?s output to decouple the load,improving the human?robot interaction although reducing the system capacities[9]. This chapter is focused on the VSAs and their potential applications in gaitrehabilitation scenarios. The first part explains the variable stiffness principle andseveral configurations and techniques to accomplish this behavior. The second partshows the T-FLEX exoskeleton?s design based on VSA, and finally, the third partpresents two experimental validations in gait assistance and stationary therapy.