Coupled, Closed-System Fluidic Actuators for Use in Wearable Rehabilitation Devices

This paper presents a novel closed-system, coupled soft actuator that aims to increase the applied bending moment that can be powered by a single pneumatic pump. The actuator incorporates both positive pressure and vacuum actuators of established design. The purpose of this development is to enable the design of an effective soft robotic wearable device for the re- habilitation of the revolute joints in post-stroke individuals. The design of a test rig to provide consistent, quantitative data on the output of the soft actuators is presented, allowing a comparison of the positive pressure, vacuum and combined (positive and vacuum) actuators. This combination demonstrates the ability to significantly increase the torque output when compared to a single actuator using the same pump for input, potentially reducing the weight of a wearable device. The closed-system, coupled soft actuator system shows opportunity for use in a wide range of applications due to this reduction in pump weight and isolation from environmental conditions. I. BACKGROUND A. Stroke rehabilitation Stroke is a medical condition which results from a loss of blood supply to the brain. The effects of stroke are wide- ranging, depending on the part of the brain affected, but often include problems with limb strength and coordination, which can be debilitating [1]. For a person recovering from stroke, getting immediate and frequent ongoing rehabilitation is key to recovery, though availability of one-to-one treatment with physiotherapists may limit this. For this reason, research has been conducted into robotic rehabilitation and many devices now exist which can assist physiotherapists. The InMotion ARM [2], MIT Manus [3] and iPAM [4] are examples of robotic devices designed to assist in stroke rehabilitation of the upper limb. Unfortunately, due to their large size, high cost and requirement for precise fitting and calibration, they are not suitable for home use and still require significant one-to-one time with a physiotherapist. B. Soft actuators A potential solution for reducing cost, size and to in- crease user comfort is the use of wearable rehabilitation devices based on soft actuators. In this paper we use soft actuators that are made of compliant materials and remain relatively compliant throughout their actuation. A class of soft actuators which lends itself to use in wearable devices is fluid-driven actuators, which are light and constructed from James Greig, Maria Elena Giannaccini and Edward Chadwick are with the Artificial Intelligence, Robotics and Mechatronic Systems Group (ARMS) School of Engineering, University of Aberdeen, Aberdeen, UK j.greig.21@abdn.ac.uk elena.giannaccini@abdn.ac.uk edward.chadwick@abdn.ac.uk relatively cheap materials. These typically use pressurised air to produce movement. Both the McKibben Actuator [5] and Bubble Artificial Muscles [6], examples of fluidic actuators, have been shown to produce forces comparable to those of human muscle for the same size and have been incorporated into rehabilitative and assistive devices. They produce a high force output, par- ticularly in proportion to their weight, though their limitation is the low stroke length which can be problematic in high range-of-motion joints such as the elbow, which is the focus of this paper. Another potential solution is the use of soft actuators which produce a bending motion, typically due to differential strain within the actuator as pressure is adjusted. There is a multitude of designs, several of which have been used in similar rehabilitation devices for the hand [7]. To determine whether this class of actuators would be suitable for the mobilisation of other joints in the upper limb, an understand- ing of the requirements for bending moment and range of motion of these in typical rehabilitation activities must first be achieved. C. Requirements Rosen et al. [8] conducted a study to determine these requirements whereby a participant conducted 24 activities of daily living (ADLs) whilst kinematic data of the arm was measured using a motion capture system. This data was then used to calculate the angle, velocity, acceleration, and torque based on a seven degree-of-freedom model. For the elbow, the maximum flexion angle was 162.9 degrees, and the maximum torque was 3.76 Nm. The maximum torque at the wrist was significantly lower, at 0.37 Nm in radial deviation and 0.25 Nm in wrist flexion. In a similar scoping study, Xiloyannis et al. give a figure of 146 degrees for the range of motion and 4.45 Nm for maximum torque at the elbow [9]. In a 1996 review of multiple studies into ranges of motion during ADLs [10], using a telephone required the maximum elbow flexion, at 160 degrees. D. Existing Devices Many stroke rehabilitation exercises consist of perform- ing specific tasks with varying levels of assistance from a physiotherapist [11], therefore the above values would be a suitable target for elbow flexion in a rehabilitation device. Some muscle strength remains, meaning the required assistance would be a proportion of this maximum torque. Another consideration is the provision of elbow extension, 2023 IEEE International Conference on Robotics and Automation (ICRA 2023) May 29 - June 2, 2023. London, UK 979-8-3503-2365-8/23/$31.00 ©2023 IEEE 10422